Food Allergies: New Challenges of Our Civilization

Abstract

People need to eat and digest food, and if they encounter a food allergy it is a real problem. Moreover, some people have a lifelong sensitization to certain products with the threat of anaphylaxis. This chapter considers different aspects of food allergies, allergenicity of dietary allergens, the significance of the gut microbiota and intestinal epithelium integrity, detailed processes of food sensitization, clinical phenotypes and management of food allergies, and, finally, mechanisms of oral tolerance. Fortunately, the gastrointestinal tract possesses robust tolerogenic mechanisms, in particular, the beneficial gut microbiota, as well as the autonomous enteric nervous system, which taken together with the gut immune cells and molecules may be called the enteric neuroimmune system (ENIS). The dual-allergen exposure hypothesis postulates that early oral exposure to food allergens induces tolerance, whereas exposure at non-gastrointestinal sites results in food sensitization and allergy development. In addition, a series of food allergic episodes does not look like a typical atopic disease and is a known exception to the rule conceived by evolution. However, the prevalence of food allergies is continuously growing, including severe cases, and it is a paradoxical problem in the face of evolution. This challenge is inherent to our civilization and will be resolved, thanks to new knowledge and technologies.

1. Introduction

The term “food allergy” is used to denote an adverse immunologic response to a food protein (allergen) and differ it from so-called “food intolerance” caused by digestive enzyme insufficiency [1]. It is estimated that 3–4% of adults and 5% of children under four years of age in industrialized and westernized countries suffer from food allergies with a broad range of polymorphic signs and symptoms. More extensive data suggest that food allergies account for even up to 10% of affected [2]. The prevalence of food allergies is continuously growing, including severe anaphylaxis caused by selected food allergens like peanuts in separate atopic individuals that can repeat for their lifetime in about 80% of them and maybe fatal [3, 4]. By contrast, food intolerance does not engage the immune system and does not lead to anaphylaxis, but it affects more than half the world’s human population.

Nevertheless, a food allergy in isolation does not look like a typical atopic disease and it is rather not a chronic atopic disease but a series of discrete allergic episodes. The gastrointestinal tract is normally a specific target organ unlike the other target organs because food components have to be used for growth and metabolism in children and renewal of the body in adults. Evolution created the gut as a tolerance zone but not a place of immune responses to nutrients. Of course, there is an enormous number of various microbes of the microbiota inhabiting the gut, and the immune system has to control the possible danger of opportunistic microbiota and pathogenic microbes, which can enter the gastrointestinal tract with food. Yet why does the immune system fight against some food proteins that lead to the disease? Undoubtedly, it is a violation of the rules conceived by evolution [5]. However, food allergies are becoming yet another problem for healthcare professionals worldwide. Furthermore, it is accompanied by a buildup of metabolic syndrome, obesity, type 2 diabetes mellitus, and chronic gastrointestinal diseases, which appear to be associated with food expansion, changed dietary behavior and preferences, new food products unknown to human natural history, instability of the gut microbiota, and, possibly, hidden food allergies based on local persistent inflammation in the gut.

Along with global changes on the planet, such as climate change, the loss of biosphere balance, a decrease in species biodiversity, SARS-Cov-2 pandemic and possible new pandemics, threat of vital resources insufficiency required for the survival of mankind, and an increase in the prevalence of food allergies represent a new challenge for our civilization and human evolution.

2. Food allergens and their allergenicity

Food allergens are a small portion among all dietary proteins. The term “allergenicity” describes the characteristic features of food allergens, which enable the sensitization, allergic inflammation, and clinical food allergies.

The well-known “Big Eight” of food allergens exhibits the strongest allergenicity and causes about 90% of all food-allergic cases. The “Big Eight” includes peanut, tree nuts, soy, wheat, cow’s milk, hen’s eggs, fish, and crustacean shellfish (see Figure 1). Allergens in cow’s milk, hen’s eggs, and wheat often lose their allergenicity when babies grow and acquire allergen tolerance. However, allergies to peanuts, tree nuts, fish, and crustacean and mollusk shellfish usually persist over a lifetime and have a high correlation with anaphylaxis [1, 3, 6].

Figure 1.

The “Big Eight” of food allergens. The food allergen group “Big Eight” includes cow’s milk, hen’s egg, wheat products, soy, peanut, and peanut-containing products, tree nuts, fish, and shellfish. The most frequency of tree nut allergy is attributed to hazelnuts, cashews, pistachios, and almonds. There are many classifications of the shellfish among them we can highlight a division of shellfish into predominant as food allergy triggers crustaceans like shrimps, crabs, and lobster, and slightly less culprit mollusks like oysters, clams, snails, and octopus.

Factors of allergen itself	Biogenic cofactors	Factor of the immune system
Primary amino acid sequence of allergenic epitopes; molecular weight lower than 70 kDa; small isoelectric point (charge); degree of protein fold; oligomerization; amount of allergenic epitopes and their proximity to each other; concentration; dose; low hydrophobicity; solubility in water; interaction with lipids; abundance in food; high stability and resistance to the extremes of food processing,; denaturation and proteolysis by digestive enzymes; and presence of intrinsic allergen biologic activities	Presence of molecular patterns and adjuvants in food	Hereditary predisposition to atopy; ability to promote the production of elevated levels of high-affinity allergen-specific IgE; route of exposure; involving innate immunity; impaired orogastrointestinal epithelial barrier; influence of the gut microbiota; sialylation of IgE; decreased enzyme secretion; and deficiency in sIgA

Table 1.

Allergenicity factors of food allergens.

In total, there are three classes of food allergens.

Class 1 food allergens (cow’s milk, peanut, hen’s eggs, etc.) are canonical oral allergens that cause sensitization through the gastrointestinal tract and display severe clinical signs.

Class 2 food allergens (e.g., carrot, celery, apple, melon, and kiwi) are cross-reactive dietary allergens with aero-allergens that trigger sensitization through the unified airway and exert less severe cross-reactions termed “oral allergy syndrome” [1, 7].

Class 3 food allergens (e.g., small food proteins less than 10 kDa, additives, contaminants, and colorants like tartrazine) with no capacity of cross-reactivity cause sensitization through the unified airway or skin and frequently result in occupational allergies [8].

The allergenicity of food nutrients, which are proteins, glyco- or lipoproteins, including novel and genetically modified food ingredients, is evaluated by many techniques such as mass spectrometry, serological assays, cell experiments, animal models, bioinformatics analysis, etc. [9, 10, 11].

Factors affecting food protein allergenicity are divided into three groups depending on (1) allergen itself, (2) biogenic cofactors, and (3) the immune system of the body (Table 1) [12, 13].

In addition, food allergens generally have to be recognized as heat-stable and heat-labile molecules. Heat-stable allergens are resistant to heat and acid and can cause systemic reactions. In contrast, heat-labile allergens are highly sensitive to heat and acid and may lead to cross-reactivity if they get into the body as pollen particles [14].

The nomenclature of food allergens [15] corresponds to the rules of the established antigen nomenclature, by which the order of letters is as follows: at the beginning, the first three letters of the genus name; next, the first letter of the species name; then the Arabic numeral of when this food allergen was identified among other allergens in this species; and after a period (.), the digits related to isoallergens. For example, an allergen of peanut, Arachis hypogaea, may be designated as Ara h 1.0101.

In atopic individuals, food allergens induce IgE antibody production by plasma cells due to type 2 helper T (Th2) cell-dependent B-cell adaptive responses, or Th2 pathway. Since 1978 [16] until the present day, the main characterization of allergens, including food allergens, is still defined by their IgE-binding frequency, which enables the division of them into major (more than 50% IgE-binding), or minor (less than 50% IgE-binding) [17]. However, this classification has become outdated because the new molecular era in allergology has already begun [18]. In the transition period of allergology natural history, two allergen generations are used by allergists for the diagnosis and allergen-specific immunotherapy (AIT):

1. natural standardized allergenic extracts, and.

2. artificial biotechnologically engineered allergenic molecules [17].

The best technique for the determination of specific IgE concentrations produced by food allergens is component resolved diagnosis (CRD) [19], which currently exists in three modifications:

1. singleplex assays (ImmunoCAP, Thermo Fisher Scientific/Phadia, Uppsala, Sweden) assess one allergen at a time; they are much cheaper and do not provide potentially unnecessary information [20];

2. multiplex (ImmunoCAP Immuno Solid-phase Allergen Chip [ISAC]) has been the preferred version for the last decade allowing simultaneous determination toward many IgE molecules [20];

3. customized allergen profiles (Euroline; Euroimmun, Lübeck, Germany) are an intermediate option, which combines allergen extracts and a set of the most relevant related allergenic molecules to evaluate the cross-reactivity, genuine sensitization, and risk profile [21].

The CRD enables an increase in the analytic sensitivity and diagnostic specificity and a decrease in potential risks, possible cross-reactivity versus primary specific sensitization. Novel CRD modifications and new technologies for the determination of sensitization are in development.

The Basophil activation test (BAT) as a functional assay displays the opportunity to indirectly detect the presence of allergen-specific IgE. After stimulation of blood basophils with an allergen and negative and positive controls, the cells are stained with antibodies linked to a fluorochrome, which allow the visualization of cells and the measurement of biomarkers CD63 and CD203c using a flow cytometer. BAT and the outcome of oral food challenges have a high correlation with food allergies [22, 23].

Skin prick testing (SPT) [24] and the more rarely used atopy patch tests [25] keep on being used as in vivo methods operated by allergists worldwide. Despite revolutionary and promising molecular methods such as CRD, allergic skin testing has to be considered as an additional, more selective, third-line diagnostic approach reserved for specific cases, such as polysensitized allergies [26].

3. The intestinal barrier and gut microbiota

3.1 The gut epithelium

The gastrointestinal tract normally represents a potent barrier for various harmful substances, allergens, pathogenic microbes, and parasites, and serves as a transit border through which input and output transport of biomolecules, water, and simple chemicals proceeds. The epithelial lining, a one-layer columnar epithelium with microvilli, linked with glycocalyx on the luminal surface contains many cell lineages among which absorptive enterocytes and colonocytes are predominant. The use of transcriptomic technology, single-cell RNA-sequencing, enabled a revisal of gut epithelium structure and description of the full landscape of cell lineages among conventional cell types (see Figure 2) [27, 28].

Figure 2.

The gut epithelium and subepithelial region. The gut epithelium landscape is currently revised due to new transcriptomic technology, the single-cell RNA-sequencing. Absorptive enterocytes in the small intestine and colonocytes in the large intestine are prevalent cell lineages. In total, the gut epithelium consists of epitheliocytes and stem cells, and many interepithelial cells perform the main function to protect the subepithelial region and internal environment against invaders and allergens. However, under certain conditions, food allergens can penetrate the epithelial barrier using one or some of four routes: (1) due to impaired epithelium integrity or leak; (2) via specialized M cells; (3) by GAP; and (4) due to uptake by long dendrites of DC. GAP—goblet cell-associated allergen passage, DC—dendritic cell, Th2—type 2 helper T cell, Tfh—follicular dendritic cell, FDC—follicular dendritic cell, ILC2, and ILC3—group 2 and group 3 innate lymphoid cells, TDC—tolerogenic dendritic cell, pTreg—peripheral regulatory T cell, TLR—Toll-like receptors.

Absorptive early, intermediate, and mature (1) gut epitheliocytes and (2) intestinal stem cells are reported to be the most numerous, whereas interepithelial (IEC) cells such as (3) transit amplifying cells, (4) bastophin 4 (BEST4+)-positive epitheliocytes, and (5) goblet cells, are shown to be in medium quantities. The remaining IEC, (6) Paneth cells, (7) tuft cells, (8) enteroendocrine (EEC) cells, (9) M cells, and (10) intraepithelial lymphocytes are identified as rare and very rare cell lineages [27, 29].

Functions of gut epitheliocytes include well-regulated absorption of nutrients and water and the barrier obstacle formation for its own microbiota, pathogenic microbes, and allergens due to adhesive interepithelial complexes composed of desmosomes, adherens junctions, and tight junctions [30]. Some known proteins, claudin, aquaporin, aquaglyceroporin provide these epitheliocyte functions [27]. Almost every week, a new epitheliocyte regenerates from stem cells in-built in the epithelial monolayer. However, cells of the epithelium can become a “gate” for food allergens and contribute to allergic inflammation. There are four routes for allergen uptake and entry into the submucosa [29]:

1. paracellular transport of allergen due to epithelium leak;

2. allergen transcytosis by M cells;

3. goblet cell-associated allergen passages (GAP);

4. direct luminal allergen uptake by long outgrowths of dendritic (DC) cells;

The simple columnar intestinal epithelium is well suited for dietary allergen delivery through GAPs since it enables fast access for allergens in a direct manner to lamina propria DCs [31]. In addition, epitheliocytes (1) constitutively express the low-affinity FcεRII (CD23) by which allergens can transcytose via the epithelium in the submucosa [29], and (2) express the pattern recognition receptors (PRR) like Toll-like receptors (TLRs) sensing food allergens and allergen-associated molecular patterns (AAMP) [32, 33].

BEST4+ epitheliocytes, a new cell lineage, are just identified [27]. Enhanced BEST4 expression on BEST4+ epitheliocytes appears to be associated with dietary consumption of sugar and fat [27].

Goblet cells, a predominant cell lineage among IECs, related to secretory cells, are the primary contributor to an additional obstacle for undesirable invaders before the epithelial barrier by secretion of high-molecular-weight glycoprotein complexes [27, 34]. In addition, they promote the process of GAP formation [35], while the GAP function may be present at other mucosal sites different from the gut. Intestinal goblet cells can perform a critical role in the capture of luminal allergens due to the GAP formation [29]. Interestingly, during interactions with DCs, goblet cells transfer these dietary allergens to DCs, which may, conversely, acquire opposite tolerogenic properties becoming tolerogenic CD103+ DCs and taking part in the proliferation of peripheral regulatory T (pTreg) cells [29, 31].

Paneth cells are rare, well-characterized columnar secretory cells, mainly located at the base of the small intestine crypts. In gut inflammation, they have also been revealed within the stomach and colon epithelium. The paneth cells release from their acidophilic granules many antimicrobial factors of neutrophil-like profile, such as α-defensins, lysozyme, IL-1β, IL-17A, TNF, etc., to provide the crypts with a sterile condition, control intestinal microbiota, and contribute to the inflammatory process [36]. The paneth cells have not yet been reported to play a role in food allergies, however, an indirect effect, through the luminal microbiota, which is regulated by these cells, is possible [29].

Tuft cells are less well studied in comparison with other ILCs. Tuft cells are able to be overactivated in relation to helminth and protist invasion, take part in promoting group 2 innate lymphoid (ILC2) cells and Th2 pathway, recognize pathogen-associated molecular patterns (PAMP) via expressed TLRs, and secrete acetylcholine, IL-25, eicosanoids, enzymes, etc. [27, 29, 37]. The ability of tuft cells to communicate with neurons is a subject for future research. There is minimal evidence for their role in food allergies, including the direct effect on food-induced anaphylaxis [29].

EECs produce over 30 neuropeptides, such as calcitonin-gene-related peptide (CGRP), vasoactive intestinal peptide (VIP), substance P, and gastrointestinal hormones [38, 39], which operate not only within the gut but communicate in the gut-brain axis [27]. Neuropeptide W secreted by EEC is known to upregulate food intake [28]. So far, there is no evidence of a functional link between EECs and IgE-mediated food allergies [29].

“Microfold” (M) cells are localized to the lymphoid follicle-covered epithelium and specialized for the uptake of particulate allergens from the lumen facilitating transcellular transport to DCs for allergen processing, allergen uploading on Class II HLA molecule grooves, and presenting to lymphocytes [27, 29, 30]. Taking into consideration the main function of M cells, it is obvious that M cells may perform an essential role in the immunopathogenesis of food allergies [29].

Conventional dendritic cells 2 (DC2s) of which outgrowths pass through the intestinal epithelium can show different phenotypes [29, 40]. At least, some of them exhibit protolerogenic properties to promote pTregs differentiation, which is important for oral tolerance. So far, in the gut, DC subsets are still insufficiently studied in humans, therefore, this information mainly comes from mouse models.

Intraepithelial lymphocytes are CD8αα + γδT cells promoting allergen transcytosis when allergens are complexed with FcεRII (CD23) expressed by epitheliocytes to get into the subepithelial region [29, 41].

3.2 The gut subepithelial region

The underlying mucosal immune system’s cells are located in the lamina propria, compressible, and elastic region, where the nourishment and functioning of the epithelium and containing immune cells, nerve fibers, glial cells, and other cells take place (see Figure 2).

Peyer’s patches, isolated follicles, and the appendix are lymphoid aggregates of the intestine, whereas the scattered lymphoid elements not organized in similar aggregates are available in the esophagus and stomach. In the aggregated lymphoid follicles, there are B-cell areas where follicular dendritic (FDCs) cells and follicular helper T (Tfh) cells promote advanced B-cell-mediated immune response. Plasma cells produce end-products: secretory IgA (sIgA), IgG, and IgE if sensitization occurs. Some allergen-specific DCs migrate via draining lymphatics to mesenteric lymph nodes, where they also trigger advanced B-cell-mediated responses. T-cells are disposed outside the lymphoid follicles in so-called T-cell zones [5].

Cell types of the subepithelial region in addition to those mentioned above are as follows: (1) DC subsets like conventional (myeloid) dendritic (cDC) cells subdivided into cDC1 and cDC2, plasmacytoid dendritic (pDC) cells, tolerogenic dendritic (TDC) cells, and inflammatory dendritic cells, (2) ILC2 and ILC3, (3) mucosal, Peyer’s patch, lamina propria, and muscular macrophages (M2), (4) mast cells and basophils, (5) eosinophils, (6) neutrophils, (7) enteric glial cells, (8) fibroblasts, and other cells.

TDCs express integrin α_E (CD103+), complexed with molecule β₇ to form α_Eβ₇ receptor for E-cadherin and essential for homing of new T cells in the gut and then to draining lymph nodes to promote pTregs differentiation [42, 43]. They also express integrin α₄β₇. The inflammatory DC subset generated from monocytes participates in many types of inflammatory processes, including allergic inflammation [44].

ILC2 and ILC3 are located in the submucosa in separation. ILC2 is known as cell activated by epitheliocyte-derived alarmins and neuromedin U and those take part in forwarding the Th2 pathway and allergic inflammation [45]. ILC3, which is more heterogeneous, activated by glial cells and VIP maintains the gut epithelium integrity due to IL-22, as well as regulates lymphoid follicle formation and oral tolerance [46].

Mucosal macrophages (M2) located close to epithelium are responsible for the survival and differentiation of epitheliocytes, intestinal stem cells, and IECs, preservation of epithelial barrier integrity, repair in its disruption, and surveillance for the gut microbiota. The other macrophage subtypes, lamina propria macrophages, Peyer’s patch macrophages, and muscular macrophages related to M2 phenotype suppress all potential immune responses [4, 47].

Mast cells are leading cells of allergic inflammation. They are heterogeneous and exist as three mast cell subsets: cells expressing tryptase and chymase (MC_TC, or “connective-tissue” cells), mast cells expressing only tryptase (MC_T, or “atypical, or mucosal” cells), and the rare mast cells expressing only chymase (MC_C) [48]. Food allergen binds to produce IgE antibodies, which interact with FcεRI on mucosal mast cells. Mast cells respond, increasing the fluid secretion, smooth muscle contraction, peristalsis, vomiting, and diarrhea due to three portions of pro-inflammatory mediators. There is also the IgE-independent alternative activation pathway of mast cells provided through the Mas-related G-protein-coupled receptor—MRGPRX2 [49]. It leads to the same effects as classical pathway.

Basophils and mast cells share the capacity of degranulating in a rapid manner and releasing histamine, but they differ in their precursors, the ability to synthesize inflammatory eicosanoids, and a particular set of cytokines and chemokines [50]. Mast cells and basophils are upregulated by IL-9 and IL-33 [51]. During recent years, new research facts concerning non-canonical functions of mast cells are accumulated, for example, participation in extracellular trapping, communication with the CNS, and less understood roles in tumorigenesis [50].

Eosinophils, analogous to mast cells, are related to main cells of allergic inflammation [52]. However, eosinophils and IL-5 likely play not such significant roles in food allergies in comparison with allergic inflammatory processes in different target organs [23, 53]. However, eosinophils are undoubtedly leading cells in another separate allergic pathology, eosinophilic esophagitis [54]. The cells have two types of granules, primary and specific/crystalloid, which contain galectin 10 (Charcot-Leyden crystals), major basic protein, eosinophilic cationic protein, eosinophilic peroxidase, enzymes, cysteinyl leukotrienes, histaminase, etc. Most of these factors release during degranulation, affect parasites in a toxic manner, and participate in allergic inflammation [55].

Neutrophils, a prevalent cell lineage among leukocytes, have long been underestimated as cells, which actively participate in allergic inflammation during the late phase [56]. They contain 200 granules of three types, larger azurophilic, smaller specific, and tertiary granules rich in a large number of pro-inflammatory mediators. In allergic asthma the Th2-low/Th17/neutrophylic endotype has been already identified, but, in food allergies, it must be described in the near future because the gut is not only a tolerance zone but the barrier target organ that is a deterrent border for a huge amount of various luminal microbes.

Enteric glial cells modulate the interactions between neurons and the immune system and maintain along with ILC3 the epithelial barrier integrity [57, 58]. Glial cells appear to orchestrate the mutual enteric neuroimmune system (ENIS).

3.3 The gut microbiota

Starting at delivery, then during childhood and all lifetime, microbiota, or microbiome, settles the gut and other barrier organs, changes its composition depending on the microenvironment, and continuously affects vital processes, preventing or promoting pathologic conditions. In this regard, microbiota is heterogeneous and can be divided into two large groups, a beneficial tolerogenic (immunoregulatory) microbiota and potentially harmful inflammatory opportunistic microbiota [59]. The tolerogenic microbiota fulfills dietary fiber fermentation and produces seven short-chain fatty acids: butyrate, propionate, acetate, formate, isobutyrate, valerate, and isovalerate, which are essential factors along with pro-tolerogenic neurotransmitters and neuropeptides for the proliferation and maturation of TDCs and pTreg cells and for the enterocytes and colonocytes regeneration due to the renewal of intestinal stem cells and inhibition of the Th1, Th2 and Th17 lymphocytes activity [60, 61]. Interestingly, the many bacteria of the first group synthesize neuro molecules, serotonin, GABA, opioids, dopamine, required for the immunoregulation in the gut and interaction with the immune and nervous systems [62, 63]. Conversely, inflammatory microbes are prone to promote the maturation of Th1 and Th17, share features with both pathogens and symbionts, and cause pathological processes under particular conditions. These two groups antagonize with each other and compete for nutrients; therefore, the role of the immune system is complex and maybe even paradoxical because it has to provide a differential approach to the gut microbiota.

Tolerogenic microbiota must meet the following criteria [5]:

1. producing metabolites that promote the allergen tolerance maintenance system at the level of the whole body at all times,

2. antagonizing with the other microbes, which may cause gastrointestinal inflammation and food allergies, and

3. dynamic, positive changes depending on the flux microenvironment.

In children, the gastrointestinal tract’s immaturity may play a role in the increased prevalence of gastrointestinal dysbiosis and food allergies seen in the first four years of life. In general, in children and adults, the main function of the gastrointestinal tract is to process ingested food into a form that can be absorbed and exploited for energy and growth, and simultaneously prevent the multiplication of undesirable microbiota in the gut. The intake of food proteins normally enables the local and systemic immune unresponsiveness in a process termed oral tolerance [2]. Dysbiosis in children is promoted by unfavorable factors, such as cesarean delivery, lack of breastfeeding, early-life-antibiotic exposure, and a low-fiber/high-fat diet. Allergen tolerance breakdown may be the end-effect of dysbiosis [64]. Adults develop dysbiosis due to diseases, genetic and epigenetic background, unhealthy diet, including a decrease in dietary fibers, vitamins, trace elements, and an increase in fat, sugar, and salt, use of junk foods, tobacco, and alcohol, as well as unhealthy lifestyle, lack of environmental sanitation, immobility, etc.

The communities of microbes comprising the gut microbiota are complex and dynamic from birth to adulthood. Factors affecting the diversity and growth of the gut microbiota show that the microbiota can dramatically influence the outcome of immune responses in the gut, including penetration of food peptides (allergens). Furthermore, this circumstance appears to be the leading cause of IgE-dependent food allergies to start or not [29, 65]. The tolerogenic microbiota is, on the other hand, a strong factor in oral tolerance maintenance at any age [66, 67, 68, 69]. However, the ratio of continuous tolerance versus food allergy episodes remains disputable, particularly why most individuals do not get sensitized during their lifetime at all [70].

In non-atopic adults, the IgE-dependent food allergies must not occur, but IgG-mediated food allergies may appear at any age if gastrointestinal disorders are available. There are also food allergic reactions, the previously so-called “pseudo allergy.” IgE-mediated food allergic reactions may present in different tissues, such as skin, gastrointestinal, respiratory and genitourinary tracts, and allergens penetrate the body in the same ways [1, 23]. However, there is not yet a full clarification why food allergies occur in only some atopic persons but not in all.

4. Enteric neuroimmune system (ENIS)

The gastrointestinal tract is a container of processing food components, digestive enzymes, metabolites, enormous microbiota, immune cells, neurons, glial cells, and immune- and neuron-derived molecules [5].

The gut is innervated by three types of peripheral nervous system counterparts under the general regulation of the central nervous system (CNS): (1) the somatosensory nervous system, (2) the vegetative nervous system subdivided into (i) sympathetic and (ii) parasympathetic divisions, and (3) the unique self-contained nervous system, termed the enteric nervous system (ENS) [71].

Sympathetic innervation of the gut is provided by efferent neurons, which are present in the cut-associated lymphoid (GALT) tissue, and act through β₂ARs expressed on most immune cells secreting preferentially protolerogenic neurotransmitter norepinephrine (NE). In the gastrointestinal tract, norepinephrine diminishes enzyme secretion, food digestion, ENS activity, gut motility, and peristalsis.

Parasympathetic innervation of the gut is achieved with cholinergic neurons, which operate using preferable pro-immunogenic neurotransmitter acetylcholine (ACh) via a7nAchRs expressed on the epithelium’s cells and immune cells, promoting the gut inflammatory process, mucus secretion by goblet and other secretory cells, and intestinal peristalsis. However, acetylcholine enables re-switching via vagus-splenic synapse with the sympathetic nervous system and displays temporary pro-tolerogenic activity termed the cholinergic anti-inflammatory pathway [72]. Notably, cholinergic preganglionic-postganglionic neuron synapses are located in ganglia placed in close proximity to the innervated intestine, whereas adrenergic preganglionic fibers form synapses with postganglionic fibers in the symphathetic trunk ganglia [57]. Both sympathetic and parasympathetic neurons are extrinsic for the ENS because their cell bodies reside in the ganglia outside the gut. The sensory fibers of the somatosensory nervous system are mainly carried by the vagus nerve [73], the major parasympathetic nerve.

The ENS is closely linked with the immune system representing, in fact, the mutual enteric neuroimmune system (ENIS). In terms of evolution, ENIS is committed to implement some vital functions, in part controversial, but strictly required for human life as follows:

• food intake, digestion, and nutrient absorption for engagement in metabolic processes in the body;

• cooperation with the gut tolerogenic microbiota and maintenance of its constancy;

• containment of the inflammatory microbiota if re-activated;

• defense against pathogenic microbes entering with food;

• autoimmune and allergen tolerance maintenance;

• preservation of intestinal epithelium integrity and control of its permeability;

• synthesis of cytokines, chemokines, growth factors, neuro molecules, hormones, vitamins, etc.;

• output of feedback signals to the CNS;

• clearance of the body.

The ENIS is composed of a huge number of intrinsic neurons and interneurons, structured in two plexuses, submucosal (disposed between the circular muscle layer and epithelium) and myenteric (located between the longitudinal and circular muscle layers), non-neuronal cells like glia, and neuro molecules affecting the gut and gut functions [57, 58, 73]. Submucosal plexus neurons control gut secretions, food component absorption, and local blood flow, whereas myenteric plexus neurons modulate smooth muscle effects [57, 58]. The ENS, immune cells, and intestinal microbiota secrete many neurotransmitters and neuropeptides among which NE, ACh, serotonin, dopamine, GABA, CGRP, [74, 75], as well as VIP, substance P, and neuromedin U (NMU) are the most significant for the gut (see Table 2 and Figure 3) [76].

Neurotransmitters and neuropeptides	Some receptors	Origin source	Prevalent activity in relation to oral tolerance	References
Norepinephrine (NE)	β2AR; αAR	Sympathetic neurons, adrenal medulla, lymphocytes, NK cells, monocytes, macrophages, EEC	Protolerogenic neurotransmitter	[57, 58, 73, 74, 76, 77, 78, 79]
Acetylcholine (ACh)	a7nAchR; M1AchR-M4AchR	Parasympathetic neurons, EEC, lymphocytes, monocytes	Pro-immunogenic neurotransmitter	[57, 71, 76, 78, 80]
Serotonin (5-hydroxytryptamine)	5-HT1-5-HT7	Central and enteric neurons, enterochromaffin tissue, EEC	Protolerogenic neurotransmitter	[72, 74, 77, 81, 82]
Dopamine	D1-D5	Sympathetic neurons, lymphocytes, DCs, macrophages, neutrophils	Predominantly pro-immunogenic neurotransmitter as well as can exert protolerogenic activity	[72, 74, 77]
γ Aminobutyric acid (GABA)	GABAA-GABAB	Neurons, T cells, macrophages, EEC	Protolerogenic neurotransmitter	[74, 77]
Calcitonin-gene-related peptide (CGRP)	CLRs	Sensory neurons, T cells, B cells, EEC, the thyroid gland	Predominantly protolerogenic neuropeptide as well as can exert ambivalent activity	[57, 71, 76, 77, 78, 79]
Vasoactive intestinal peptide (VIP)	VPAC1-VPAC2	Parasympathetic sensory neurons, EEC, the gut	Predominantly protolerogenic neuropeptide as well as can exert ambivalent activity	[57, 58, 71, 73, 76, 77, 78, 79, 83]
Substance P	NK1R-NK3R	Sensory neurons, microglia, lymphocytes, DCs, macrophages, eosinophils	Pro-immunogenic neuropeptide	[57, 72, 78, 79]
Neuromedin U (NMU)	NMUR1-NMUR2	Parasympathetic sensory neurons, EEC	Pro-immunogenic neuropeptide	[57, 58, 73, 76, 78, 79]

Table 2.

The most significant neurotransmitters and neuropeptides in the gut.

Figure 3.

Enteric neuroimmune system (ENIS). The ENS as a part of combined ENIS forms two plexi, submucosal and myenteric, and contains intrinsic neurons and nerve fibers of sympathetic, parasympathetic, and somatosensory nervous systems of which cell bodies are located outside the ENS. Most sensory fibers are carried by the parasympathetic fibers. Some secreted in the ENIS neuro molecules exert pro-immunogenic effects, in particular, ACh upregulates mucus production, GAP formation, and mast cell degranulation; SP inhibits pTreg proliferation; NMU triggers ILC2 activation and Th2 pathway. In contrast, NE displays protolerogenic activity downregulating ILC2 and Th2 cells, and upregulating M2 polarization of muscular macrophages. CGRP and VIP show ambivalent and even paradoxical effects, for example, CGRP acts on ILC2 in a controversial manner concerning cytokine production, but downregulates M cell development. VIP promotes Th2 pathway and, simultaneously, pTreg cell proliferation and along with glial cells epithelium integrity. ENIS—enteric neuroimmune system, ENS—enteric nervous system, ACh—acetylcholine, SP—substance P, NMU—neuromedin U, NE—norepinephrine, CGRP—calcitonin-gene-related peptide, VIP—vasoactive intestinal peptide, GAP—goblet cell-associated allergen passage, DC—dendritic cell, Th2—type 2 helper T cell, ILC2 and ILC3—group 2 and group 3 innate lymphoid cells, pTreg—peripheral regulatory T cell, TLR—Toll-like receptors. Pro-immunogenic effects are noted in green, and protolerogenic effects are noted in red.

Neurotransmitters and neuropeptides serve as the main instrument by which the ENIS controls the homeostasis and all functions in the gut. As you can see in Table 2, neuro molecules are presented as predominantly protolerogenic or predominantly pro-immunogenic, and some of them can exert ambivalent activity. In healthy conditions, the summarized potential of neurotransmitters and neuropeptides in the gut is protolerogenic and anti-inflammatory, but in food allergies, conversely, it is prone to allergen tolerance breakdown and allergic inflammation [5]. Some facts of how neuro molecules influence the human immune cells in the gut allergic inflammatory process are continuously accumulated. However, our knowledge of neuronal-gut immune cell units as a new neuroimmunology paradigm comes mainly from mouse models [58, 84].

NE inhibits ILC2 activity, Th2 pathway [57, 73, 76, 78], and induces anti-inflammatory M2 phenotype of muscular macrophages [58, 73, 74].

ACh causes goblet cells to secrete mucus, form GAP [76] and amplifies the degranulation of mast cells [80].

Glial cells via neurotrophic factors cause ILC3 to produce IL-22 for the maintenance of the gut epithelium integrity [57, 58].

CGRP acts on ILC2 in a paradoxical manner inhibiting IL-13 secretion and activating IL-4 synthesis [76], but, in total, CGRP downregulates Th2 pathway of immune response responsible for allergic inflammation in the gut [79], including CGRP effect due to M cell downregulation [76].

VIP stimulates IL-22 production by ILC3 [76], and proliferation of pTregs [83], but, on the other hand, promotes Th2 pathway, migration, and survival of Th2 cells [78, 83].

SP upregulates DCs migration, Th1 pathway [72], and inhibits pTreg proliferation in the gut [79].

NMU activates ILC2 and causes them to produce IL-5 and IL-13 [73, 76, 79] that upregulates Th2 pathway of immune response and allergic inflammation.

Most mechanisms of allergic inflammation cannot be explained only by the participation of immune cells and immune-derived molecules. Food allergies are such a case. Since allergic inflammation disrupts homeostasis not only in the gut and other target organs but also in the whole body, neuronal control is extremely necessary. Neuro molecules produced by neurons and non-neuronal cells display short-term life but long-term effects in relation to a place of allergic inflammation and beyond. So far, although numerous studies of neuroimmune interactions in food allergies are the subject of discussion, our comprehension is still incomplete. Taking into account the significance of the subject, this knowledge may become a novel source of updated therapeutic approaches to food allergies in the near future.

5. Sensitization to food allergens and allergic inflammation

There are oral, skin, and respiratory routes of entry of environmental allergens, including food allergens, into the body [1]. Although modern experimental and clinical studies support a role for skin exposure to dietary allergens, which initiate the sensitization and initiation of the Th2 pathway B-cell response, the oral route remains important for food allergies [30].

Experimental studies are reported to highlight the significant role of IECs in the facilitation of dietary allergens to penetrate the epithelial barrier and trigger IgE sensitization [30]. When allergens appear in front of the epithelium, they use any of four transcytosis routes (see Figure 2 in unit 3) [29] to get into the subepithelial region rich in various cells, including those required for triggering allergic responses. Accordingly, the gut epitheliocytes generate as a “danger signal” particular cytokines called alarmins, IL-25, IL-33, and thymic stromal lymphopoietin (TSLP), upregulating three types of cells: ILC2, allergen-presenting DCs, and Th2 cells. Activated ILC2 produces IL-5, IL-9, and IL-13 influencing over eosinophils and mast cells. Food allergens are engulfed by DCs, processed, uploaded on class II HLA molecule grooves, and as allergen/HLA II complexes presented to lymphocytes, which are activated after recognition and involved in clonal expansion and maturation. Differentiated Th2 cells upregulate the Th2-mediated B-cell response with IgE end-production and memory B and memory T cells formation [3]. Memory about the current allergen becomes lifelong. The whole process proceeds in the lymphoid follicles as well as draining lymph nodes. Interestingly, intestinal epitheliocytes can constitutively turn into allergen-presenting cells like DCs and present allergens to lymphocytes [23].

Th2 pathway activation is called “the type 2 cytokine storm,” which stimulates expansion of the main cells of allergic inflammation, mainly mast cells and basophils. Th2 cells produce IL-4, IL-5, IL-9, IL-13, and IL-33, which promote allergic inflammation. Notably, IL-5 does not play a leading role in food sensitization, in contrast to responses to other allergens, in distinct target organs [23]. IL-33 is reported to be essential for the maturation of mast cells [51]. Another immunoregulatory T-cell subset, Tfh cells, secretes IL-21, IL-4, and IL-13, important for IgE class switching due to recombination and somatic mutations in B cells, maturing plasma cells and growing allergen-specific IgE affinity. The degree of involvement of Th9 cells in food sensitization is disputed [85].

Since protolerogenic neurotransmitters and neuropeptides are prevalent in the ENIS, food allergens cannot easily overcome the system of allergen tolerance maintenance. However, if it occurs, allergic inflammation commences, and clinical food allergies develop [5].

Allergic inflammation is an immunopathological process, which proceeds in three phases: (1) early phase, (2) late phase, and (3) chronic allergic inflammation [56]. The early phase usually gets started within 2–3 h after uptake of a causative food allergen depending on absorption in the gut or less if in the oral cavity and includes the release of normally preformed mediators of mast cells [50] and basophils, such as histamine, serotonin, chemotactic peptides for neutrophils and eosinophils, and enzymes (chymase and tryptase). These mediators affect the nerve cells causing smooth muscle contraction, mucus production by goblet cells, increased capillary permeability, recruitment of neutrophils and eosinophils. In some cases, mast cells release a wide range of chemicals in greater quantity than usual, causing reactions collectively known as anaphylaxis in multiple body areas, including the unified airway, cardiovascular system, brain, etc. Frequently, it may be a life-threatening condition [86].

The late phase in food allergies develops in 6–9 h and later. Two groups de novo produced after activation of mast cells and basophils neoformed and neosynthesized mediators consist of cysteinyl leukotrienes (LTC₄, LTD₄, and LTE₄), prostaglandin D₂ (PGD₂), platelet-activating factor (PAF), pro-inflammatory cytokines, chemokines, growth factors, nitric oxide, and C3 and C5 components of complement [87]. These biomolecules act on surrounding tissues promoting the inflammatory process. Endothelial cells express those adhesion molecules, which facilitate the involvement and activation of neutrophils, eosinophils, inflammatory DCs, and monocytes from the blood into the site of the allergic inflammation. The eosinophils release various inflammatory molecules, including major basic protein, eosinophilic cationic protein, IL-5, etc. The involved Th2 cells secrete cytokines among which Il-4, IL-13, and IL-33 are the most potent and affect plasma cells, promoting IgE isotype switching. So, the events acquire long-term potential [5].

The process becomes chronic allergic inflammation after repeated exposures to food allergens and continuous recruitment of inflammatory cells releasing numerous pro-inflammatory mediators. In food allergies, in the gut, local persistent allergic inflammation is inherent though clinical signs may manifest only from time to time.

6. Food allergies as particular atopic conditions

The natural history of food allergies contains attempts to explain why this type of allergy occurs. Some hypotheses have been proposed and are still being discussed [2, 88].

The hygiene hypothesis and “old friends” hypothesis. The absence of exposure to microbes and allergens in early childhood may increase predisposition to allergic sensitization due to the underdevelopment of the immune system promoting Th2 polarization rather than Th1.

The dual-allergen exposure hypothesis. In infants, if the skin barrier function is impaired, exposure to environmental food allergens causes allergen sensitization through the skin rather than via oral route. Food allergies are likely generated as a combination of both skin and gut exposure to food allergens, with a preferable tendency towards sensitization through the skin route. Proponents of the hypothesis attempt to put the idea of early introduction of allergenic food for susceptible babies into practice [6, 89].

The vitamin D hypothesis. Vitamin D (cholecalciferol) is a recognized immunomodulatory and protolerogenic substance of which deficiency may lead to possible risk factors for food allergies. Low concentration of vitamin D is reported to increase the risk of peanut allergy, decrease the differentiation of pTregs, and activate Th2 polarization.

The microbiota hypothesis. The presence of particular bacterial strains, their metabolites, and some dietary substrates may promote the development of food allergies.

However, so far there is insufficient evidence to confirm or prove all these conceptual interpretations [2].

Food allergies are characterized by polymorphic clinical signs, which may manifest in any body system, particularly in such target organs as the gut, skin, unified airway, and genitourinary tract. They include swelling of the lips, tongue, or larynx, hives, skin rash and itching, bronchospasms, difficulty swallowing, feeling sick or vomiting, abdominal pain, or diarrhea, angioedema, etc. Swelling of the larynx may be life-threatening due to shortness of breath and even respiratory arrest.

Food allergies are reported to develop in different phenotypes such as classic, cross-reactive, aerosolized, and α-Gal syndrome (mammalian meat allergy), among which basic atopic sensitization due to IgE overproduction is predominant [90]. Besides phenotypes, food allergy endotypes, persistent, transient, local and systemic reactions, and drugs/exercise/alcohol-induced forms are described [90].

According to [91, 92], there are the following phenotype groups of food allergies:

1. IgE-mediated group (classically, of type I hypersensitivity by Gell and Coombs [93]), which includes acute urticaria and angioedema, allergic asthma, delayed food-induced anaphylaxis to mammalian meat, oral allergy syndrome, gastrointestinal allergic immediate reaction, and food anaphylaxis; this group is subdivided into (1) primary (with sensitization to Class 1 food allergens) and (ii) secondary food allergies (with hypersensitivity to Class 2 food allergens, for example, oral allergy syndrome);

2. Mixed group: atopic dermatitis and eosinophil-associated gastrointestinal inflammatory disease;

3. CD4+ T-cell-mediated group (classically, of type IV hypersensitivity by Gell and Coombs [93]) that includes celiac disease [41], food protein-induced enterocolitis syndrome (FPIES) [94], food protein-induced proctocolitis, and food protein-induced enteropathy.

In general, IgE-dependent food allergies are prevalent according to the classification.

It is fitting to highlight that phenotype is a recognizing feature of a disease, such as morphological, physiological, or biochemical property, or behavior, with no implication of a cell/molecular mechanism. Endotype represents a different physiological or pathological approach, which involves and uncovers cell and molecular mechanisms of a disease and response to therapy. Both phenotype and endotype are dependent on genotype and epigenetic modifications as well as environmental factors [2, 90, 95].

In the opinion of Chong et al. [86], most food allergy studies are devoted to either not persistent cases of food allergies or anaphylaxis. So far, such phenotypes as oral allergy syndrome (or pollen food allergy syndrome) [1, 7, 96], sporadic mild or moderate food allergies, and not truly life-threatening and really severe life-threatening anaphylaxis may be considered relatively studied [86]. Eosinophilic esophagitis [54] based on a high level of eosinophilic inflammation is a separate, strong genetically associated allergic disease [91]. Also, other rarer food allergy conditions than prevalent IgE disorders are outside of the scope of this chapter.

The cross-reaction if oral allergy syndrome occurs can be explained by the structural similarity of allergens, which may be found in both pollens and food. The list of cross-reactions is large and continuously expanding. Common examples are sensitization to birch, elm, and alder linked with food allergy to apple, peach, cherry, tomato, carrot, etc., and hypersensitivity to ragweed associated with a food allergy to watermelon, banana, zucchini, cucumber, etc. In addition, allergy to grass is associated with hypersensitivity to honey, orange, melon, etc. [6]. Chitinases are a group of allergens often found in plant food (wheat, rice, tomato, raspberry, grape, banana, coffee, etc.), latex (hevein), arthropods like house dust mites (HDM), and insects (silkworm). Accordingly, chitinases develop cross-reactivity syndrome and may even lead to anaphylaxis [97]. Most people can become tolerant of the cross-allergy if food products containing heat-labile allergens have been baked, cooked, and roasted.

There is currently no explanation for why life-threatening anaphylaxis occurs in only some atopic individuals among those who are allergic to food allergens [51, 98]. Genetic and epigenetic factors in food anaphylaxis are of high interest and are directly and indirectly involved in IgE-mediated food sensitization. Genetic background plays a significant role in the manifestation of most atopic diseases, whereas epigenetics matters greatly through three epigenetic mechanisms: DNA methylation, covalent posttranslational histone modifications, and micro-RNA-mediated gene silencing. Therefore, it may be essential for the interactions between various susceptibility genes, epigenetic modifications, immunologic processes, nerve impulses, and environmental factors [99]. However, explicit monogenic mutations linked with only anaphylaxis have not been found, but separate facts about some mutations causing anaphylaxis-like conditions and metabolic disturbances have continuously been accumulating [91]. It is likely that patients prone to more severe food allergies and also poorer outcomes in oral AIT have a specific phenotype [86], which is not yet confirmed by fundamental research findings.

In general, the series of prerequisites why life-threatening anaphylaxis occurs in only some individuals among those who are sensitized to food allergens is as follows [5]:

• polygeneous hereditary predisposition to atopy;

• individual hypersensitivity to selected allergens;

• dose and pathway of allergen entry specific for a patient;

• repeated penetration of the same allergen, which already caused anaphylaxis;

• acquired genetic alterations and epigenetic modifications promoting the weakness of the allergen tolerance maintenance system;

• peculiarities of forming memory T and B cells to food allergens [3].

Anaphylaxis in food allergies is observed more often than in drug- and insect venom-induced cases and shows some particular features important for differential diagnosis (see Table 3).

Criteria	Food anaphylaxis	Drug- or insect venom-induced anaphylaxis
Common manifestation age	Children	All ages
Prevalent signs and symptoms	Respiratory	Cardiovascular
Predominant route of allergen penetration	Oral, skin, or respiratory	Parenteral
Common onset time	Within 2 hours after exposure to allergen	Rapidly
Death cause	Respiratory arrest	Cardiac arrest
Mast cell tryptase as biomarker [100]	Moderate positive or negative	Positive

Table 3.

Peculiarities of food and nonfood-related anaphylaxis ([86], modified).

Food allergies frequently coexist with other atopic diseases, such as atopic dermatitis, allergic rhinitis, and allergic asthma. In comparison with children without food allergies, children sensitized to food allergens are two to four times more likely to suffer from asthma, particularly poorly controlled asthma [101]. Intake of snails in patients allergic to HDM can exacerbate the course of severe asthma, and airborne allergens such as wheat, fish, and seafood may result in so-called “food-induced asthma” [102]. Children co-sensitized to food and aero-allergens suffer from more severe clinical signs of allergic rhinitis [103].

However, a food allergy in isolation does not look like a typical atopic disease and it is rather not a chronic atopic disease but a series of discrete allergic episodes. That is because the gastrointestinal tract is a specific target organ unlike the other target organs being a zone allergen tolerance; therefore, food allergies may be a known exception to the rule conceived by evolution [5].

7. Diagnosis and management of food allergies

The diagnosis of a food allergy is very complex, requiring a detailed past medical history (or allergy anamnesis), physical examination, CRD [19, 104], BAT [22], SPT [24], and repeated visits to an allergist. Allergic skin tests, particularly SPT, occupy a central place in the diagnosis of food allergies. However, if a skin test is not suitable for revealing the food sensitization, a specific IgE determination is recommended [92]. An oral food challenge (or controlled food provocation test) may be administered, which an allergist conducts in the allergist’s office taking into consideration the risk of an unpredictable severe allergic reaction like anaphylaxis. Nevertheless, the oral food challenge currently represents the “gold standard” diagnostic test due to the difficulty of food allergy diagnosis [105]. In relation to all diagnostic allergy tests, the main rule is that they must be used in combination, be guided by the medical history and be clinically relevant [92].

Treatment management of food allergies [106] consists in educating the patient about allergen avoidance, prescribing pharmacotherapy [92], biologics [106, 107], and AIT [108, 109, 110, 111]. Urgently in anaphylaxis, an epinephrine auto-injector must be used. Once the diagnosis of food allergy is confirmed, strict elimination of the causative food allergen from the nutrition is absolutely necessary during lifespan; this allergen source can be replaced with another food product or, at least, if a causative allergen is heat-labile, be processed by baking, roasting, or boiling. Treatment of acute food reactions involves the prescription of epinephrine, antihistamines, glucocorticosteroids, and bronchodilators. Biologics can be applied as monotherapy or as adjuvant therapy to AIT.

AIT is a single method in IgE-mediated food allergies, which is disease-modified, well-documented, effective, and high-level medicine-based treatment option leading in allergen tolerance establishment [23, 92, 112]. In food allergies four routes of AIT were historically used: subcutaneous, sublingual, oral, and epicutaneous. Unfortunately, research into subcutaneous AIT in food allergies displayed severe systemic adverse effects, therefore many observations had been discontinued. Nevertheless, the subcutaneous route with peanut allergoid exhibited better tolerability [113]. Sublingual AIT displayed clinical efficacy and good tolerability but to a smaller extent than those in oral AIT, however, in a study with the use of standardized birch pollen extract better efficacy was described [114]. Epicutaneous AIT in patients with peanut allergies is currently undergoing a clinical trial.

Nowadays, research into the oral route of AIT in food allergies is actually at the cutting-edge. At the beginning of AIT (the up-dosing period) the daily oral administration of small but gradually increasing amounts of food allergen is conducted under medical supervision. The causative allergen is in-taken with food, and physical activity is avoided 2 hours after [115]. After completion of the up-dosing period, a daily maintenance dose may be in-taken at home. The oral route of AIT induces desensitization, irrespective of whether achievement of persistent tolerance is not yet evident. In the course of oral AIT, mild and moderate adverse reactions may be frequent, for example, mouth or throat itching and swelling, abdominal pain, but the risk of anaphylaxis and eosinophilic esophagitis remains. In 2020, Food and Drug Administration (FDA) approved the first licensed oral AIT product for peanut allergy—Palforzia® [116]. However, Palforzia® cannot be used in untreated or uncontrolled asthma and existing problems related to the esophagus and the gut. The European Academy of Allergy and Clinical Immunology (EAACI) prepared the guidelines on AIT for IgE-dependent food allergies. Trials have found substantial benefits for cow’s milk, hen’s egg, and peanut allergies, but a better adverse effect profile and high efficacy for oral AIT with cow’s milk and hen’s egg have not yet been confirmed. However, low-dose AIT may be useful in children with severe cow’s milk allergy [117], but allergens has to be administered with caution to patients with a history of anaphylaxis [118]. AIT with food allergens should be exclusively performed in clinical centers with significant experience in such immunotherapy that patients should frequently visit during the up-dosing period. Patients must also make an informed decision about the therapy [119]. The dual-allergen exposure hypothesis is a precise reproduction of one of the mechanisms by which food allergies may develop; therefore, the hypothesis has been studied and discussed most extensively [119]. Furthermore, it is based on fundamental immune tolerance theory [120], which, if clinically experienced can be a good rationale for food allergy prevention using the approach of early introduction of food allergens in babies.

Prevention of food allergies by early introduction of food has been disputed. Some researchers suggest that the early introduction of peanut, cooked eggs, cow’s milk, sesame, white fish, and wheat in exclusively breastfed infants at the age of 3 months with a hereditary risk of developing food allergies would reduce the prevalence of food allergy by the age 3 [6]. The idea of early introduction of allergenic food corresponds to the dual-allergen exposure hypothesis [89, 121], but, so far, it did not show clinical efficacy in relation to all those food allergens [88, 122].

Since food allergy patterns are different in distinct countries, for example, wheat allergy is prevalent in Japan and Thailand, whereas shellfish hypersensitivity is predominant in Singapore and the Philippines, the study of early introduction of potentially allergenic food has to be continued in children at high risk [123, 124]. In fact, the early introduction of food allergens during food intake is a natural induction of allergen tolerance, while AIT is an artificial medical approach.

Natural recovery from food sensitization is possible in infants depending on the allergen source, for example, cow’s milk, hen’s eggs, and wheat. Reverting a food allergy to allergen tolerance is the main purpose of AIT and is characterized by a loss of Th2 cells and an increase in Th1 cells, the simultaneous induction of blocking IgG antibodies, and suppression of inflammation’s effector cell functions [23].

8. Oral tolerance

The logical completion of the chapter is a reference to oral tolerance, a state, which has to be recovered if a food allergy occurs. Simultaneously, oral tolerance enables the discussion of all questions related to food allergies and connected to food allergies as a challenge at present time.

This health condition is an organ-specific form of allergen tolerance, which is, in turn, a particular form of immune tolerance. However, there is not any separate oral tolerance since it exists in terms of combined allergen tolerance. In general, tolerance represents an antipode to active immune responses, which leads to the production of effector T cells and immunoglobulins participating in an “immune battle” against “non-self” or “former self”. When an invader is defeated, the immune response has to complete the turning into tolerance. If tolerance does not develop, many types of pathology may manifest. Food allergies are a particular case because food proteins, glycoproteins, and lipoproteins are not dangerous invaders at all, and the immune response to them occurs by error. However, food allergies exist exhibiting a growing prevalence, and can become life-threatening, therefore, from an evolutionary viewpoint, oral tolerance maintenance has to be a solution to the challenge.

Oral tolerance must meet the following main criteria:

1. a potent allergen tolerance maintenance system in the gut,

2. balance in the intestinal microbiota with a significant amount of useful tolerogenic microbes, and.

3. integrity of the gastrointestinal barriers.

Oral tolerance depends on multiple factors, which can maintain or destabilize it under the daily penetration of food proteins, instability in the gut microbiota, changing signals from neuro molecules, and continuous trafficking of pro-inflammatory cells and molecules. The dual-allergen exposure hypothesis confirms a classical postulate of immune tolerance since the time of Burnet [121] that tolerance depends on age. If atopy predisposed babies before 3 months of age begin to consume peanuts this diet could decrease the risk of a peanut allergy after 12 months. In contrast, when such infants do not consume peanuts the likelihood of food allergy increases in the near future [6, 43]. Another important factor for oral tolerance breakdown is a high continuous dose of food allergens to which exposure may be available because of food intake and dietary preferences are daily and permanent.

In addition, impaired epithelial barrier integrity of the mouth predisposes a person to food allergies, in particular profilins-containing allergens, such as vegetables, fruits, seeds, and plant-based products [125]. Frequently disregarded mouth pathologies appear to present the prerequisites for food allergy manifestation confirming the existence of the oral route for rapid penetration of food allergens, which some researchers prefer to consider only as supplementary and even rare. Meanwhile, a healthy oral cavity is very significant in terms of food allergy prevention. Inflammatory processes in the gut result in increased epithelial permeability, facilitate food components to meet the submucosal immune cells and, under a deficiency of tolerogenic activity, trigger a Th2 pathway adaptive response or other IgE-independent pathogenetic mechanisms of food allergies [2, 23, 29, 88]. In sections 3 and 4, we described the high significance of the tolerogenic microbiota and ENIS in oral tolerance maintenance.

In the face of food allergies and inflammatory processes in the gut, evolution created the natural system of long-lasting oral tolerance maintenance enforced by the following components [5]:

• TDCs, including intestine-specific CD103+ TDC [4, 42];

• allergen-specific FoxP3 + pTreg cells [126, 127], as well as type 1 regulatory T (Tr1) cells and type 3 helper T (Th3) cells [128, 129, 130];

• regulatory B (Breg) cells [127, 128] and blocking antibodies;

• Peyer’s patch and lamina propria M2 macrophages [4, 47];

• immunosuppressive cytokines: IL-10, transforming growth factor β (TGF-β), and IL-35 [5, 42, 43];

• pro-tolerogenic neurotransmitters and neuropeptides [5, 73, 79];

• tolerogenic microbiota [59, 60, 61, 62, 63].

Intestinal CD103 + DCs commonly endocyte food allergens penetrating through the epithelial barrier in readiness to promote Th2 pathway, however, metabolic products of tolerogenic microbiota, such as short-chain fatty acids and retinoid acid assign tolerogenic properties to these DCs turning them into CD103 + TDCs. TDC migrate into the mesenteric lymph nodes where they induce naïve T cells to mature into FoxP3 + pTreg cells using TGF-β and retinoid acid [4] and tolerize allergen-specific effector lymphocytes, making them anergic. Mature pTregs using expressed chemokine receptor CCR9 and integrin α4β7 arrive in the gut subepithelial region and along with TDC contribute to allergen tolerance to food allergens.

TDC and pTreg cells [42, 126, 131, 132, 133] play a general role in oral tolerance acting in a synergic manner to provide (1) the synthesis of IL-10, TGF-β, IL-35, and enzymes producing toxic for effector lymphocytes derivates like kynurenines; (2) expression of coinhibitory molecules, such as PD-1, CTLA-4, BTLA, and LAG-3 [134] known as antagonists of costimulatory molecules required for immune response forwarding; (3) competition with proliferative lymphocytes for the essential growth factor IL-2 [132]; (4) inhibition of Th1, Th2, Th17, and Th22 pathways [132] and many inflammatory cells like ILC2; (5) activation of follicular regulatory T (Tfr) cells [135], which downregulate Tfh cells. Subsets of pTregs are Tr1 and Th3 cells specialized for the mucosal barrier sites acting in a pTreg-like manner using the same mechanisms. Tr1 cells are known as FoxP3⁻ IL-10-secreting Tregs [130], whereas Th3 cells are recognized as FoxP3⁻ TGF-β-secreting Tregs [129]. In addition, there are pTreg subsets functionally directed to certain helper T cell subpopulations [129, 132] and memory pTregs [126, 127, 132].

B cells also generate a regulatory subset called Bregs. Breg cells contribute to oral tolerance secreting protolerogenic cytokines like IL-10, TGF-β, and IL-35, which upregulate IgG₄ production and downregulate IgE synthesis by plasma cells [136].

Peyer’s patch and lamina propria M2 macrophages originate from circulating monocytes and exert tolerogenic properties by secretion of IL-10 and synergistically functioning along with pTregs [4].

IL-10 is the most potent immunosuppressive cytokine. In terms of the effects of TDC, pTregs, and other above-mentioned cells with tolerogenic properties, IL-10 inhibits the Th1 and Th2 pathways, expression of costimulatory molecules on allergen-presenting cells and lymphocytes, and activity of many inflammatory cells [42]. More or less, TGF-β and IL-35 display the analogous properties as IL-10.

GABA and serotonin are the most potent protolerogenic neurotransmitters in the ENIS with very rare exceptions, whereas neuropeptides CGRP and VIP can exhibit ambivalent effects depending on the microenvironment (see Table 2 and Figure 3 in Section 4). Nowadays, research into the influence of neuro molecules over oral tolerance is currently at the cutting-edge.

In the ENIS framework, the tolerogenic microbiota implements a unique role [61] in deterrence of the opportunistic bacteria growth and generation of metabolites and neuro molecules for TDCs and pTregs proliferation without which the gut and the whole body cannot exist since there are many other forms of gastrointestinal activity and gut-based vital functions.

In summary, oral tolerance to food and its loss occurs from a complicated interaction between the allergens in the food, the microbiota inhabiting the gut, intestinal epithelium integrity, immune and non-immune cells in the GALT, and protolerogenic neurotransmitters and neuropeptides found in the ENIS. If oral tolerance breaks down, the state may be recovered using the therapeutic approach called AIT with food allergens.

9. Conclusions

Food allergies are characterized by some routes of food allergen penetration and polymorphic clinical signs, which may manifest in anybody system, not only through the gut and in the gut. Normally, the intestine is a zone of tolerance in contrast with a place of immune responses to nutrients because evolution created the gastrointestinal tract as a container for food digestion, inhabitance of tolerogenic microbiota, and source of neuro molecules, hormones, and cytokines derived from the enteric neuroimmune system (ENIS).

The gut epithelium is a complex barrier membrane between the intestinal lumen and subepithelial region rich in cells and molecules of the ENIS. Single-cell RNA-sequencing, a transcriptomic technology, allowed to describe the novel composition of epitheliocytes and interepithelial cells, which were unknown in past [27, 28]. Nowadays, research on neuroimmune regulation of the gut has acquired a particular significance [76]. Nevertheless, food allergies caused by global changes on the planet and the new living environment for mankind are a violation of the rules conceived by evolution [5] and challenges for human civilization.

Allergen-specific immunotherapy (AIT) with food allergens can become an efficient approach for therapeutic management of this increasing pathology. Researchers have begun to describe the molecular structure of food allergens and have performed chip-based assays for many allergens. A study of the structure of culprit food allergens has allowed engineering synthetic and recombinant vaccines for AIT [1]. In addition, the idea of early introduction of allergenic food in infants, which corresponds to the dual-allergen exposure hypothesis [89, 121], is, in fact, another perspective approach for food allergy prevention [6, 123, 124].

Funding

This chapter received no external funding.

Conflict of interest

We confirm there are no conflicts of interest.