Human Blood

Abstract

The human blood is composed of 3 layers of humors when separated into its different components. The component that is clear, slightly yellow (plasma), the whitish viscous-like (buffy coat) and the red fluid (red cells). The plasma component is composed of proteins; however, it will not be discussed in this chapter. The buffy coat is composed of white blood cells and platelets. The white blood cells are composed of granulocytes and agranulocytes; all of which take part in immune defense. The granulocytes, including monocytes have non-specific immune response while agranulocytes, which include B and T cells have specific immune response. The platelets function to help maintain normal hemostasis during vascular injury. Blood group antigens are found on the surface of red cells and are composed of proteins, carbohydrates and lipids. They are mostly inherited on autosomes with the exception of two which have been found to be inherited on the X chromosomes. With the advance of technology, some of their physiological functional roles have been elucidated. These include; structural integrity, cationic exchange, transporters, adhesion and receptor functions, and cell to cell communication. However, these mechanisms have been capitalized by infectious agents to gain entry to the human body causing disease.

1. Introduction

Since ancient times, blood has been viewed as the very essence of human life. In fact, description of the human blood dates back to the writings of Hippocrates in about 400 B.C. He described it as being composed of four layers of fluid; one that looked “black bile, red blood, whitish viscous-like (phlegm) and yellow bile” [1]. This was later clarified in the twelfth century by a Swedish physician as a description of blood that is undergoing clotting process, where blood is being separated into distinct portions. During this time and earlier, the state of health and disease were thought to have been caused by an imbalance between these layers of blood. This helped to explain why bloodletting was performed during those ancient times and into the nineteenth century [1, 2].

During those early years (200 AD), red blood was taught to be the dominant humor and therefore bloodletting was carried out to reduce excess blood from circulation, to slow down the heart rate, and also to reduce an inflammatory process in an individual suffering from an inflammation. It was believed that this process would in turn bring balance to the different layers of blood, and ultimately health to the individual being bled [2].

The composition of blood was however not known, until the discovery of the microscope in 1673 by Anton Leeuwenhoeck. By 1683, Leeuwenhoeck could see minute microorganisms such as bacteria using the instrument [3] and even accurately described and measured red blood cells [1]. During this time, bloodletting was based on unscientific principles and therefore remained controversial.

However, with the advance of technology at the turn of the twentieth century, new approaches and standardized methods were developed, which contributed to better understanding of the composition and structural organization of red cells [4]. This eventuated in the current acceptance of Bloodletting as a therapeutic treatment for specific chronic diseases associated with hematochromatosis (iron overload), erythrocytosis (elevated hematocrit), porphyria and polycythemia (excess number of red cells) [3].

As a result of advance in technology, there was also the understanding that red blood cell membranes are composed of protein and lipid residues, which define their structural composition, physiological and biological roles. Much of their protein content is made up of hemoglobin; essential for transportation of oxygen (O₂) to tissues and carbondioxide (CO₂) to the lungs. Apart from transportation, they have other physiological functions such as; maintaining structural integrity of the cell, modulation of cell-cell interactions or vascular endothelium-cell interactions, anchorage site of cystoskeleton, anion exchange, adhesion and receptor functions [4]. It is now well understood that, apart from these normal physiological functions, red cells also serve as biological mediums by which external invaders can enter blood circulation and tissues to cause disease to the human body [5]. Their pathophysiological roles are discussed in Section 4, Subsection 4.2.

2. Blood Components

2.1 Composition

Blood does not only contain fluid but also other substances such as proteins, carbohydrates, lipids and antigens [4]. The blood is comprised of 55% plasma and 45% formed or cellular elements [6]. The formed or cellular elements include buffy coat, which contains white blood cells (WBCs) and platelets, and red blood cells (Figure 1). Blood is essential in humans and other animals and performs multiple tasks. The erythrocytes or red blood cells contain hemoglobin and function in the transport of oxygen (O2) to tissues and carbon dioxide (CO₂) from tissues to the lungs. The WBCs (Leucocytes)) are involved in the body’s defense against the invasion of foreign antigens. The Platelets (thrombocytes) are involved in the process of hemostasis; prevention of blood loss during injury [6, 7, 8].

Figure 1.

Blood, when centrifuged, it is separated into 3 different layers; plasma (contains proteins), Buffy coat (contains mainly white blood cells and platelets), and red cell concentrate) [7].

2.2 Synthesis of blood components

All human blood goes through a process known as hemopoiesis or hematopoiesis. Hematopoiesis or hemopoiesis is the process of blood production. This process proceeds through different stages starting from early embryonic life (mesoblastic stage), to the hepatic stage, and then to the myeloid stage. The process in the embryonic and early foetal life occur in the yolk sac, although at this stage, only the very early red cells (erythroblasts) are formed. As the foetus develops, all blood cells are formed in the foetal liver. Apart from the yolk sac, hemopoiesis also occurs in the mesoderm of intraembryonic aorta/gonad/mesonephros (AGM) region. After several weeks, it also occurs in the spleen, lymph nodes and thymus. From the third to the 4th month until birth, blood cell production occurs exclusively in the bone marrow. As the child matures into adulthood, blood cell production is confined to only the flat bones such as; the sternum, ribs, iliac bones, vertebrae, and proximal ends of long bones (Figure 2) [7, 8].

Figure 2.

The phases of hematopoeisis. During the process of blood production, erythroblasts are the first to be formed in the yolk sac, followed by the rest of the cell lines in the fetal liver, then in the spleen and finally in the BM from the 4th month of life onwards to adulthood [7].

2.2.1 Red blood cells (Erythrocytes)

Mature red blood cells (RBCs) or erythrocytes are very small cells with a diameter of about 8.0 μm. They maintain a biconcave discoid shape and do not contain a nucleus [9]. The life span of mature RBCs is about 120 days. After the 120 days, they are phagocytosed by reticuloendothelial system (RES) macrophages in the spleen [8]. Their main function is to transport O₂ from the lungs to various tissues and organs and from these tissues and organs, it carries CO₂ to the lungs for reoxygenation [7, 9, 10].

2.2.2 White blood cells

White blood cells are categorised as granulocytes and agranulocytes. Granulocytes include; neutrophils, eosinophils and basophils, and agranulocytes include; lymphocytes and monocytes [9, 11].

2.2.2.1 Granulocytes

These are also called polymorphonuclear leucocytes because their nuclei are oddly shaped and their cytoplasm have densely stained granules when stained with Leishman’s stain or Wright-Giemsa stain. They play a major role in immune defence as part of the innate immune system. Their role in defence is non-specific, short-lived and without memory [11].

2.2.2.1.1 Neutrophils

Neutrophils make up 97% of the granulocyte lineage and are the first to arrive at sites of infection. The size of a mature neutrophil is about 10 to 12 micrometres (μm), with 2 to 5 lobes of deep purple nucleoli when stained with Leishman’s stain. They also have very fine light pink cytoplasmic granules. These granules contain proteins and enzymes such as; lysozyme, lactoferrin, vitamin B₁₂-binding protein, myeloperoxidase, acid phosphatase, elastases and others. Their major roles are; phagocytosis of bacteria, viruses and yeasts, formation of Neutrophil Extracellular Trap (NET), degranulation and cytokine production [11, 12]. Their lifespan is 1–2 days in peripheral blood circulation [7, 8].

2.2.2.1.2 Eosinophils

Eosinophils are small bilobed granulocytes, with granules that stain red orange with the Wright-Giemsa stain. These granules contain proteins and enzymes such as the major basic protein, cationic proteins, peroxidase and histaminase. These are used to defend against helminthic parasites. On activation, eosinophils produce debilitating toxic respiratory burst and also create transmembrane plug that kill their target. Their maturation in the BM takes 2 to 6 days and their lifespan in blood is less than 8 hours [7, 8, 9].

2.2.2.1.3 Basophils

Basophils are the least numerous of the circulating WBCs. Their nucleus contains condensed chromatin, shrouded by darkly stained coarse granules when stained with the Wright-Giemsa stain. These granules contain inflammatory mediators and proteins such as; histamine, serotonin, heparin, Major Basic protein, and enzymes such as DNAases, proteases and lipases. They also express receptors to IgE and therefore have the ability to become activated when bound to IgE-Antigen immune complexes. On activation, they degranulate releasing their content that kill their target [12]. They also play a role in hypersensitivity reaction. Their lifespan is less than 3 days [13].

2.2.2.2 Agranulocytes

These cells do not contain multiple lobes like the granulocytes.

2.2.2.2.1 Lymphocytes

There are two types of lymphocytes. These are B and T lymphocytes. They are the major players in the adaptive immune response against foreign invasions. They constitute 20–30% of the total WBC population. Unlike the granulocytes, their actions are slow, specific and have memory [12]. The B cells function to produce antibodies and the T cells function to provide help to B-cells for antibody production, kill off virus-infected cells, and also play regulatory roles. The sizes of these cells range from 8 to 10 μm [7, 8, 9]. Naïve lymphocytes live longer in their restful state than effector lymphocytes. Lymphocytes that have differentiated into memory cells have longer lifespan. The different lifespan periods are dependent on heterogeneous populations during stages of differentiation and activation [14].

2.2.2.2.2 Monocytes

Monocytes are usually large in size, measuring about 12–20 μm in diameter. The nucleus is generally kidney-shaped with fine chromatin. They have abundant cytoplasm which appear blue, and often contain azurophilic granules and vacuoles. They circulate in blood for about a day before they exit to tissues where they are called macrophages or histiocytes [12]. In the blood, their function is to protect against bloodborne pathogens. In tissues, their major roles are phagocytosis, antigen presentation, cytokine production and NET formation [7, 8, 9]. This population of cells comprise a heterogeneous population; distinguishable by their cell surface markers and functions. A blood classical monocyte’s lifespan is ~1.0 day, a blood intermediate monocyte’s life span is ~ 4.3 days and a non-classical blood monocyte’s lifespan is ~7.4 days [15].

2.2.3 Platelets

Platelets are the products of cytoplasmic fragmentation of megakaryocytes in the BM in a process called megakaryopoiesis [12]. The diameter of platelets is about 2–3 μm. They contain α-granules, dense granules, and lysosome. Their main function is their synergistic interactions with endothelial wall and plasma proteins to maintain normal hemostasis during vascular injury. They remain alive in the blood for about 10 days [7, 8, 9].

3. Blood group antigens

The term “blood group” generally refers to a person’s collection of “red cell surface antigens”. These are found on the surface of red blood cells as part of the red cell membrane. They are made up of proteins, glycoproteins and glycolipids. These may elicit an immune response in individuals lacking these antigens.

Blood group antigens have been classified by the International Society for Blood Transfusion (ISBT) into 30 blood group systems [16, 17, 18]. However, as of June, 2019, there are currently 38 blood group systems (Table 1), with over 200 red cell antigens classified under these systems, while some are classified as collections [16, 19]. These antigens are inherited and as such, understanding of their genetic makeup is important as well as their unique characteristics which differentiates one group or one antigen from another. Such characteristics include; genetic expression, structure and location on red blood cells, and the type of antibody they induce. The most common and clinically significant of these blood group antigens are the antigens belonging to the ABO and Rhesus blood group systems because they have the potential of eliciting an immune response that can cause fatal consequences through blood transfusion and or pregnancy [16].

3.1 Blood group antigen inheritance

Genes are units of inheritance that encode particular proteins needed for production of particular inherited traits. Genetic information caried in these genes are found in double stranded deoxyribonucleic acid (DNA) called chromosomes. Humans have 23 pairs of these, of which; 22 are autosomes and 1 pair of sex chromosomes. Within these chromosomes are sites called genetic loci; where genes are located. Within these genetic loci are alternate forms of genes called “alleles” [16].

Inheritance of blood group antigens follow an autosomal codominant pattern of inheritance, where alleles inherited from both parents are equally expressed on autosomes [16]. Hereditary patterns of these antigens are based on the Mendelian principles of inheritance; which sprouted from early experiments done on pea hybrids [17]. His genetic concepts of Dominance, Independent Segregation and Assortment are currently applied in understanding inherited characteristics (traits) observed in human blood group genetics. For each inherited trait (character), there are two alleles inherited from each parent. The expression of this inherited trait is dependent on the combination of the two alleles inherited. One of these alleles suppresses the effect of the other, while the other allele can only be observed in the absence of the dominant allele. The allele that suppresses the expression of the other is called the dominant gene, while the other that is not expressed in the presence of the dominant gene is called recessive. This concept is referred to as the Law of dominance [16, 17].

Thus, alleles inherited from each parent can be the same (homozygous) or different (heterozygous). The concept of independent segregation refers to each parent having a set of alleles for a particular trait, either of which can be passed onto the next generation. These alleles segregate, allowing for only one allele to be transmitted to an offspring. For example, using the letters “A” and “a” to represent a dominant and a recessive allele respectively, there are four possible combinations (Table 2). The AA combination constitutes entirely of AA (homozygous) offspring and the aa combination comprise entirely of aa (recessive) offspring. The offspring of the aa allele combination differ from the AA, aA and Aa combination due to the absence of a dominant allele. The offspring of Aa and aA (heterozygous) gene combinations inherited traits common to all four combinations [17].

Traditional name	ISBT No.	Symbol	Traditional name	ISBT No.	Symbol
ABO	001	ABO	Gerbich	020	GE
MNS	002	MNS	Crower	021	CROM
P1Pk	003	P1	Knobs	022	KN
Rh	004	RHD, RHCE	Indian	023	IN
Lutheran	005	LU	Ok	024	OK
Kell	006	KEL	Raph	025	RAPH
Lewis	007	Le	John Milton Hagen	026	JMH
Duffy	008	FY	I	027	I
Kidd	009	JK	Globoside	028	GLOB
Diego	010	DI	GIL	029	GIL
Yt	011	YT	RHAG	030	RHAG
Xg	012	XG	Forssman	031	Fors
Scianna	013	SC	Jr	032	Jrᵅ
Dombrock	014	DO	Lan	033	Lan
Colton	015	CO	Vel	034	Vel
Lansteiner–Wiener	016	LW	CD59	035	CD59.1
Chido/Rodgers	017	CH/RG	Aug	036	Aug1
Hh	018	H	Kanno	037	Kanno
Kx	019	XK	Sid	038	Sid

Table 1.

International society for blood banks (ISBT) classification of the known blood group systems with their ISBT numbers and symbols [19].

Parents	A	A
A	AA	Aa
a	aA	Aa

Table 2.

This Punnett square describes Mendel’s concept of “independent segregation” using symbolic “A” and “a” to denote inheritance of a dominant allele and a recessive allele respectively [17].

Mendel’s third concept is “independent assortment”. During meiosis, a mixture of genetic material is produced resulting from random behavior of genes on separate chromosomes. These genes are inherited independent of each other on different chromosomes but are expressed on the same red cell membrane. Figure 3 illustrates the ABO and the Kell blood group system genes, whose genes are located on chromosomes 9 and chromosome 7 respectively [16].

Figure 3.

Independent assortment. The ABO blood group antigens are sorted independently from the Kell antigen genes because they are inherited on different chromosomes. However, they can all be expressed on the same red cell membrane separately and discretely [16].

Genes coding for most of these blood group antigens are inherited on autosomes except the Xg and the Kx. The Kx is coded for by the Xk gene, while the Xg is coded for by the Xgᵅ allele, both of which are located on the X chromosome. This means that fathers having the latter genes would pass it on to their daughters and none to their sons. If, however, mothers have these genes, they would pass it on to both genders [16, 20].

Inheritance of blood group antigens follow distinct patterns of inheritance. Some genes code for antigens that are co-dominant., some are dominant over another and some are recessive. For example, in the ABO blood group system, the A and B antigens are codominant; both alleles are expressed to show the trait. When A and O alleles, or B and O alleles are inherited, the O trait is not expressed because the A and the B alleles are dominant over the O. When O and O alleles are inherited, the O traits are observed in the absence of a dominant gene. The O gene is said to be recessive (Figure 4) [17].

Figure 4.

Strength of immunogenicity of the Rhesus Blood Group antigens in decreasing order of immunogenicity.

According to the ISBT, some of these blood group antigens have been classified as blood group systems based on their serologic and molecular characteristics (Table 1) and their locations on specific chromosomes has also been elucidated (Table 3).

Blood group system	Chromosome	Blood group system	Chromosome
Rh	1	Kidd	18
Duffy	1	Lewis	19
MNS	4	Landsteiner–Wiener	19
Chido/Rodgers	6	Lutheran	19
Kell	7	Hh	19
ABO	9	P	22
Kx	X	Xg	X

Table 3.

Some of the blood group antigens and their chromosomal locations, all of which are found on autosomes and only two are found on the X (sex) chromosomes [16].

3.2 Structure and location on red cells

Three of the basic structural properties of red cells are hemoglobin, enzymes and the membrane [18].

3.2.1 Hemoglobin

Although mature red cells no longer have nucleus nor mitochondria, they have an abundance of hemoglobin, a red pigment that contains oxygen. The hemoglobin carries oxygen to all parts of the body to keep the body alive and collects carbon dioxide (CO₂) from the tissues to the lungs for re-oxygenation.

3.2.2 Enzymes

The Embden-Meyerhof pathway is a metabolic pathway used by mature red cells to generate energy through a series of enzymatic pathways that catalyze the conversion of glucose to lactate and pyruvate. Within this pathway, there is a shunt called the Rapoport Luebering shunt that generates the production of 2,3Diphosphoglycerate (2,3DPG); important in influencing the release of O₂ in tissues. Apart from production of energy and 2,3 DPG, another end product of this glycolytic pathway is generation of Nicotinamde Adenine Dihydrogenase (NADH); necessary for reducing nonfunctional methemoglobin to oxyhemoglobin. Another metabolic pathway in red cells is the pentose phosphate shunt. Two enzymes are generated during this process. These are called glucose-6 phosphate-dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase (6PGD), which then generate Nicotinamide Adenine Diphosphodehydrogenase (NADPH). This enzyme with Glutathione reductase catalyzes the generation of glutathione (GSH), which along with glutathione peroxidase function to detoxify hydrogen peroxide and thus rendering the red cell safe from oxidant damage [18].

3.2.3 Red cell membrane

The red cell membrane is comprised of proteins, lipids and carbohydrates. Blood group antigens are found linked to these on the red cell membranes. Some of these red cell antigens are only found on the red cell membrane or in fluids, while some are found both on the red cell membranes and in body fluids. Those found on the red cell membrane and also in fluids are; the ABO blood group system, Hh and Lutheran. In fact, the ABO antigens are also found in lymphocytes, platelets, epithelial cells and endothelial cells and the kidney having been adsorbed from the plasma [16]. Those found on red cell membranes include; the Rhesus, Kell, Kx, Duffy and Kidd. There is one known blood group system that is unique from the others in that it is found predominantly in fluids. This blood group system is called the Lewis and it is produced by tissue cells and released into body fluids. However, these get adsorbed onto red cell membranes shortly after birth and continues on for the first 6 years of life [19]. Although it is not clinically significant, serologic antibody detected may indicate H. pylori infection as the Leᵇ allele has a receptor to this gram- negative bacteria [16].

Like the ABO blood group system, the Le gene does not code directly for its antigen but instead codes for a glycosyltransferase called L-fucosyltransferase, which then adds an immunodominant sugar called L-fucose to the precursor substance (H antigen) to form the Leᵅ antigen or Le (a+) phenotype. Its adsorption onto red cell membranes depends on the presence of the precursor substance (H antigens) on red cells and also presence of the L-fucosyltransferase type 1. Conversion of the Lewis antigen from Leᵅ allele to Leᵇ in secretions depends on the Secretor gene (Se). If the individual also had inherited this gene, which is mainly found in fluids, then the L-fucose is added to the precursor substance in fluids to form the Leᵇ antigen or Le(b+) phenotype. This then gets adsorbed onto red cell membrane preferably over the Leᵅ antigen [16, 20].

3.2.3.1 The ABO, rhesus and other blood group antigens

Genes coding for the formation of some of these antigens do not code directly for their respective antigens. Instead, they code for glycosyltransferases, which in turn catalyze the transfer of immunodominant sugars from donor molecules to a precursor substance for the formation of their respective antigens. Examples of these are; the H, ABO, Se, Lewis, I/i and P₁. The H, ABO and secretor (Se) blood group antigens are inter-related, in that the H antigens forms the basis for the formation of the A and B antigens on red cells and in secretions [20]. Lack of formation of the H precursor antigen on red cells result in the formation of the Bombay (OH) blood type, instead of the ABO [21]. These individuals lack the ABH antigens and thus possess naturally occurring antibodies against the A, B, O and H antigens in their plasma. This exposes them to fatal consequences during blood transfusion. The Se gene on the other hand play a vital role in the formation of ABH antigens in secretions. This is because it controls the expression of the H antigen in secretions [21]. In individuals who lack the Se genes, there is no formation of the H antigen in secretion and ultimately no formation of the A and B antigens in secretions as well.

There are however, cases where an individual inherits a dominant allele and a recessive (Sese) or co-dominant alleles (SeSe) from each parent in secretions but lacks the H gene on red cells. In such individuals, formation of the H antigen will still occur and thence formation of A and B antigens in secretions but not on red blood cells. These individuals are said to have the “para-Bombay” phenotype [22]. Some amounts of the A and B antigens may adsorb onto red cell membranes from plasma and hence are detected on red blood cells [22]. This is summarized in Table 4.

Inherited	Genes		Antigenic expression
			On red cells	In saliva (secretions)
AB	HH	SeSe	A, B, H	A, B, H
AB	HH	sese	A, B, H	None
OO	HH	SeSe	H	H
OO	HH	sese	H	None
oh	h	sese	None	None
para-oh	h	SeSe	None	H, some amounts of A, B antigens adsorbed from plasma
para-oh	h	Sese	None	H, some amounts of A, B antigens adsorbed from plasma

Table 4.

Interaction between the ABO, H and Secretor genes depicting the expression of soluble antigens on red blood cells and in secretions [16, 22].

Some of the blood group antigen genes that code directly for the formation of their respective antigens include the Rhesus, Kell, Kidd, Duffy and MNS. Among these, the Rhesus antigens are very immunogenic because they are protein in nature, the most immunogenic of which is the Rh D, followed by the c, E, C, e. (Figure 4). Initially founded in 1939, the Rh blood group system is the most complex and polymorphic with about 50 well-defined related antigens assigned to its system classification by the ISBT [8, 23]. It is mostly associated with Haemolytic Disease of the Foetus/New Born (HDFN) especially in a second pregnancy of a Rh D positive child conceived in a mother who does not have the Rh D antigen. In the general population, 85% have the Rhesus D antigen, while the rest are negative for it [19, 24].

However, the Kidd, Kell and Duffy are all considered clinically significant as well because they have also been implicated in Hemolytic Disease of the Newborn and therefore recognition of antibodies against these antigens are vital in relation to blood transfusion and pregnancy [16].

After the Rhesus antigens, the Kell blood group antigens are next most immunogenic. It is associated with another blood group system called the Kx, inherited on the X chromosome. Absence of this on an individual’s red cells weakens the expression of the Kell antigens, a condition called “Mcleod phenotype”. They develop red cell abnormalities such as acanthocytosis and reticulocytosis [16, 20].

The Duffy Blood group antigens are glycoproteins found on chromosome 1. These glycoproteins span the membrane of the red cells. First defined in 1950 in a patient who was suffering from hemophilia, it is best remembered for its association with malaria. Individuals who do not possess this antigen (Fy a-b-) are protected against Plasmodium vivax and Plasmodium knowlesi infections. This blood type is common amongst African and American Blacks [16, 25]. Like the Duffy, the Kidd blood group antigens are glycoproteins but located on chromosome 18. They are not as polymorphic as the Rhesus and Kell blood group antigens. They play a role in urea transport. They have also been implicated in causing extravascular haemolysis in a delayed type of hemolytic transfusion reaction [16, 20].

Like the ABO and Lewis blood group antigens, the I blood group antigens are oligosaccharides, which along with the i antigen, exist on the ABH oligosaccharide chain precursors nearer to the red cell membrane. The i antigen on the other hand has not been assigned to a blood group system and remains as a collection. The i antigen is mostly expressed on red cells of New Born and cord blood, while the I is mostly seen in adults red cells [16, 18]. The P1PK Blood group antigens are glycoproteins and glycolipids, and like the ABO, Lewis, and I blood group antigens, they are also formed through the actions of glycosyltransferases. At birth, P1 is poorly expressed. The MNS are structurally glycoproteins; their sugar components are primarily composed of sialic acids attached to proteins which lends the negative charge of red cells [20].

3.3 Antibody response to blood group antigens

Karl Landsteiner’s discovery of the ABO blood group antigens in 1900 [16, 26] was the beginning of safe blood transfusion as we know today. He began by first experimenting with his own blood and then that of his co-workers. When he began to mix serum taken from co-worker A with red cells from co-worker B, he realized that these formed clumps. When he then mixed his serum with red cells from both of his two co-workers, he recognized that he had antibodies against both. This he appropriately called blood group O, which to become the universal donor. The other two co-workers A and B, he called anti-A and anti-B respectively because their serum agglutinated when mixed with each other’s red cells. In his article published in 1900 on these experiments, he added a footnote that stated “the serum of healthy humans has an agglutinating effect, not only upon animal blood cells, but frequently upon blood cells from other individuals as well” [26].

The antibodies against the ABO blood group antigens are the most clinically significant because they are pre-existing. Based on Landsteiner’s rule, healthy individuals have antibodies against antigens that they do not have. This is the basis for all blood transfusions today. Patients’ blood is always typed and crossmatched before they are infused to avoid fatal intravascular hemolytic transfusion reactions. An individual with blood antigen A has antibodies against the B antigen and an individual with blood antigen B on their red cell membrane has pre-existing antibodies A in their plasma. Individuals with no AB antigens on their red cells have both A and B antibodies in their plasma, while those that have both A and B antigens on their red cells, they do not have pre-existing antibodies in their plasma (Table 5).

Blood type (antigens)	Antibody-A	Antibody-B	Antibodies-AB
On red cells	In plasma	In plasma	In plasma
A	NIL	Present	Present
B	Present	NIL	Present
AB	NIL	NIL	NIL
O	Present	Present	Present

Table 5.

Based on Karl Landsteiner’s conclusions, healthy individuals have antibodies in their plasma against antigens that they do not have.

Antibodies against the ABO blood group antigens are mainly of the IgM class and therefore are capable of reacting at temperatures ranging from 4°C to 22°C or room temperatures. Because of its pentameric structure, it is able to bind to 10 of these red cell antigens in at any one time. This is enough to trigger off a massive intravascular complement protein reaction via the classical pathway resulting in an acute hemolytic transfusion reaction if the wrong ABO blood is transfused to an individual with a different blood type. This type of reaction usually occurs within minutes or hours of transfusion of the wrong blood. If not stopped quickly, fatal consequences like disseminated intravascular coagulation (DIC), irreversible shock and death can occur. Antibodies against the Luᵇ antigens in the Lutheran Blood group system are also clinically significant. Although rare and mainly of the IgG class, it has been found to be associated with transfusion reactions and HDFN [16].

Auto antibodies against the P antigen is bi-phasic and it is called the Donath-Landsteiner antibody. It is referred to as bi-phasic because it is able to bind to the P1 or P2 antigens at lower temperatures especially in the extremities of the body and when warmed to warmer temperatures, the complement cascade is activated resulting in haemolysis. These antibodies are mainly associated with paroxysmal cold hemoglobinuria; a rare disorder characterized by cold associated haemolysis and hematuria [16].

Autoantibodies against the I are mainly associated with patients with Mycoplasma pneumoniae infections and cold haemagglutinin disease. In these patients, strong auto-agglutinations are observed in in-vitro analysis. In patients with disease conditions such infectious mononucleosis, lymphoproliferative disease and sometime in cold hemagglutinin disease, anti-i is usually detected.

4. Blood group antigens as modes of disease transmission

For hundreds of years, the red cells were thought to be inert; with no form of biological functions. This was never been justified until in 1865, when Hoppe-Seyler discovered that in the red cells, there is an abundance of hemoglobin. These red cells have two important properties that allow them to squeeze easily through blood capillaries supplying tissues with oxygen to keep these tissues alive [1]. These are flexibility of its membrane and fluidity of its content. Any imbalance in these two properties will cause reduced survival of these red cells and removal hence by macrophages in the spleen [20].

4.1 Physiological functions of red cell antigens

Apart from the important roles in O₂ transport to tissues and CO₂ back to the lungs, production of ATP, 2,3 DPG and production of enzymes that ultimately catalyzes the biochemical processes that result in the reduction of the dysfunctional methemoglobin to oxyhemoglobin and reduced GSH, red cell antigens located on and across the red cell membranes also play other roles in many ways. Some of their known physiological functions include, gylcosyltransferases, structural maintenance of red cells, protein transportation, complement pathway molecules regulation, adhesion molecules and as microbial receptors [16, 23]. These functions are summarized in Table 6.

Blood group antigens	Putative functions on red cell membrane	Blood group antigens	Putative functions on red cell membrane
ABO	Glycosyltransferases	Chido/Rodgers	C4 complements adsorbed onto red cell membrane
MNS	Contributes to glycocalyx. GPA likely acts as a chaperone for Band 3	Hh	Fucosyltransferase
P	Glycosyltransferase	Kx	Xk protein linked to Kell glycoprotein. Homology to neurotransmitter transporters.
Rh	Involved in CO₂/O₂ or NH₄ᶧ/NH₃ transport or maintenance of cell shape	Gerbich	Glycophorins C & D. Could link to glycocalyx. Links membrane to cytoskeleton
Lutheran	Binds laminin 511 and 521. Probably adhesion/receptor in erythropoiesis	Cromer	Decay-acelerating factor. Inhibits activity of C3 convertase. Also protects cell from lysis by autologous complement
Kell	Possibly processes endothelin 3	Knops	Complement receptor 1. Binds and processes immune complexes
Lewis	Not synthesized on red cell membranes	Indian	Binds hyaluronan. Probably adhesion/receptor
Duffy	Antigen receptor for chemokines, possibly used for removal from peripheral blood	Ok	Probably adhesion/receptor
Kidd	Urea transporter	Raph	May associate with integrins to generate laminin-binding complexes
Diego	Band 3 anion exchanger 1. Exchanges HCO₃ᶧ/Cl-. Links membrane to cytoskeleton	John Milton Hagen	Probably adhesion/receptor
Yt	Its function is unknown	I	Glycosyltransferase involved in branching of oligosaccharide chains
Xg	Possibly adhesion/receptor	Globoside	Galactosytransferase
Scianna	Possibly adhesion/receptor	GIL	Water and glycerol channel
Landsteiner–Wiener	Intracellular adhesion molecule-4. Binds integrin. Possibly adhesion /receptor involved in stability of erythroblastic islands	RHAG	Rh associated glycoprotein probably involved in CO₂/O₂ or NH₄ᶧ/NH₃ transport

Table 6.

Putative physiological functions of some of the known red cell antigens [22].

4.2 Pathophysiological functions of blood group antigens

Blood is a pharmaceutical therapy for treatment of various blood component deficiencies and blood loss. However, it is quite often forgotten that, apart from its normal biological functions, it also serves as vessel for transmission of various blood borne pathogens. It is now well documented that some blood group antigens have been found to be associated with increased susceptibility to infections [27] enhance disease progression in others [28], while in others have indicated reduced susceptibility and severity [29]. Susceptibility to infection often depends on the geography and epidemiology of the different blood group antigens [30].

Among the ABO blood group antigens, the A antigen has been shown to be associated with increased mortality from the COVID-19 than Blood groups B and O [30]. With Blood groups B and AB, they have a higher risk of suffering from thromboembolism caused by the COVID-19 infections than O because they have higher levels of von Willie Brands Factor (vWF) [31]. Blood group A has also been reported to play a synergistic effect with the Hepatitis B virus (HBV) on the risk of development of pancreatic cancer [32]. Blood group A is also found to be significantly associated with the HBV infections, while syphilis was significantly associated with the Rhesus blood group in the same study [33]. Data from one study demonstrated that Blood group B antigens are associated with lower risk of being infected with hepatitis B virus (HBV) while Group O has been demonstrated to have had a 12% risk of being infected with HBV [32]. Severity of diarrhoea caused by Esherishia coli, Vibrio Cholerae and Helicobacter pylori is dependent on the O blood type and Secretor status of an individual [34]. Blood group AB is associated with the severity of dengue disease in secondary infections [27]. Like the covid-19 infections, blood group O plays a protective role against severe malaria infection [35, 36]. Being a non-secretor also play a role in reducing risk of infection by the HIV-1 and also slows down disease progression. However, on the other extreme, being a Secretor promotes infections by Haemophilus inflenzae, Neisseria meningitidis, Streptococcus pneumonie and Esherishia coli [34].

Helicobacter pylori is associated with peptic ulcer, disease, gastric carcinoma and the Norwalk virus, however disease progression is enhanced in the presence of Leᵇ antigens as these serve as receptors to the bacteria [16]. Furthermore, autoantibodies against the I (autoanti-I) are increased in Mycoplasma pneumoniae infections and Cold haemagglutinin disease. Additionally, autoantibodies against the i antigens (autoanti-i) are elevated in infectious mononucleosis, lymphoproliferative disease and also in Cold Hemagglutinin disease. The Duffy blood group antigens on the other hand play a protective role against Plasmodium vivax (Pv) invasion [24]; a parasite species that causes malaria. This applies only to individuals who does not express the Duffy antigens (Fya-b-).

5. Conclusion

The human blood is the very essence of life as it supplies the whole body with O₂ and nutrients needed for its sustenance of life. The blood group antigens are a part of this scared suspension of fluid that flows throughout the body unendingly throughout life. Carried in its membranes are the structures that can serve two purposes in a normal physiological sense and pathophysiological, in that apart from carrying out functions that sustains the livelihood of the body that carries it, it also serves as a means of entry for foreign invaders, which cause an imbalance in the normal physiological functioning of the body causing disease state.

Acknowledgments

We wish to acknowledge the contribution of Mr. Gairo Gerega of the University of Papua New Guinea School of Medicine and Health Sciences for supplying some notes on the blood group antigens.

Conflict of interest

“The authors declare no conflict of interest.”