ABO ANTIGENS AND ANTIBODIES

The ABO blood group system is based on the presence or absence of specific carbohydrate structures on the surface of red blood cells and the presence of corresponding antibodies in the plasma. The major ABO specificities are A, B and H.

The importance of the ABO system comes from the close relationship between its antigens and antibodies. A person whose red cells carry A antigen normally has anti B in the plasma. A person whose red cells carry B antigen normally has anti A. A person whose red cells carry both A and B antigens normally has neither anti A nor anti B, while a person whose red cells carry neither A nor B antigen normally has both anti A and anti B.

This relationship is fundamentally different from many other blood group systems because ABO antibodies are normally present without deliberate exposure to incompatible red cells. As a result, an ABO incompatible red cell transfusion can produce a very rapid and severe immune reaction.

The ABO system is therefore central to immunohaematology. Understanding the system requires a clear understanding of two related components: the antigens present on the red cell and the antibodies present in plasma.

ABO Antigens

An antigen is a substance capable of being recognized by a specific antibody or immune receptor. In the ABO system, the antigens are carbohydrate structures located at the outer surface of the red cell membrane.

ABO antigens are not simple proteins. They are oligosaccharide structures attached to larger molecules, particularly glycoproteins and glycolipids within the red cell membrane. The carbohydrate chains project outward from the membrane, allowing antibodies and other molecules to interact with them.

The specificity of an ABO antigen depends mainly on the terminal carbohydrate structure. A and B antigens are closely related chemically, but a difference in their terminal sugar gives them different antigenic specificities. This relatively small structural difference is sufficient for the immune system to distinguish A from B.

The H Substance

The H substance is the basic precursor from which A and B antigenic structures are produced.

Formation of H substance involves the addition of fucose to an appropriate precursor carbohydrate chain. The resulting H determinant provides the structure that can subsequently be modified to produce either A or B specificity.

The process can therefore be understood as a sequence:

Precursor carbohydrate

→ H substance

→ A or B determinant

The H substance is important because A and B antigens are not constructed independently. They are produced by modifying the existing H structure.

In the formation of A antigen, N acetylgalactosamine is added to the H structure. In the formation of B antigen, galactose is added to the H structure. When the H substance is not converted into A or B determinant, it remains the major ABO related structure expressed by group O red cells.

A antigen is the carbohydrate structure that gives red cells A specificity.

The immunodominant terminal sugar of the A determinant is N acetylgalactosamine. It is added to the terminal portion of the H substance by an A specific glycosyltransferase.

The important point is that the A antigen is not simply N acetylgalactosamine by itself. The terminal sugar forms part of a larger carbohydrate structure, and its particular arrangement gives the complete structure its A antigenic specificity.

A antigen is expressed on the red cells of individuals belonging to an A expressing ABO phenotype. It is also expressed on the red cells of individuals who express both A and B.

Because ABO antigens are carbohydrate structures, their expression is determined by the activity of enzymes that add specific sugars to precursor carbohydrate chains.

B antigen is the carbohydrate structure responsible for B specificity.

The immunodominant terminal sugar of the B determinant is D galactose. This sugar is added to the H substance by a B specific glycosyltransferase.

The structural difference between A and B is therefore mainly the terminal sugar that is added to the H precursor.

A antigen has terminal N acetylgalactosamine.

B antigen has terminal galactose.

This difference is small at the chemical level but highly important immunologically. Anti A recognizes the A determinant, while anti B recognizes the B determinant.

In individuals expressing both A and B, both types of determinant are present on the red cell surface.

Group O red cells do not express A or B determinants, but this does not mean that they lack ABO related carbohydrate structures.

They retain H substance.

In fact, because the H substance is not extensively converted into A or B determinants, group O red cells generally have a relatively large amount of H substance on their surface.

This is an important distinction:

Group O means absence of A and B determinants, not absence of H substance.

The H substance is therefore the underlying structure from which A and B antigens can be produced, while remaining as the major ABO related determinant on group O red cells.

Distribution of ABO Antigens

ABO antigens are most important on red blood cells, but their expression is not restricted to erythrocytes.

ABO related carbohydrate structures can also be expressed on epithelial and endothelial cells. Depending on the tissue and the individual's biological characteristics, related structures can occur in body secretions and on other cell surfaces.

The antigens are carried on different glycoproteins and glycolipids. The exact carbohydrate chains carrying the ABO determinants can differ between tissues.

This wider distribution is important because ABO antigens are therefore better regarded as histo blood group antigens, rather than structures found exclusively on red blood cells.

Antigen Density and Expression

The number of antigenic determinants present on a red cell can influence the strength of an antigen antibody reaction.

Antigen density refers broadly to the amount of a particular antigenic structure expressed on the cell surface. A red cell with a greater number of available antigenic sites provides more opportunities for antibody binding.

The strength of a visible agglutination reaction, however, is not determined by antigen density alone. Antibody concentration, antibody affinity, temperature, reaction conditions and the physical arrangement of antigenic sites can also influence the reaction.

This is why the presence of an antigen and the strength of the reaction produced by that antigen should not be regarded as exactly the same thing.

Introduction to ABO Antibodies

ABO antibodies are immunoglobulins found in plasma that recognize A and B antigenic determinants.

Their distribution follows a characteristic pattern. Individuals normally produce antibodies against the major ABO antigen that is absent from their own red cells.

Thus, anti A is found in individuals whose red cells do not express A antigen, while anti B is found in individuals whose red cells do not express B antigen.

This reciprocal relationship is essential to understanding the ABO system.

The antibodies are often described as naturally occurring antibodies because they are commonly present without previous transfusion or pregnancy. However, naturally occurring does not mean that they appear without any immunological stimulus. Their development is associated with exposure to environmental substances, particularly microbial carbohydrate structures that resemble ABO antigenic structures.

Anti A

Anti A is an antibody directed against the A antigen.

It is normally present in the plasma of individuals whose red cells do not carry A antigen.

Therefore, anti A is normally found in the plasma of group B and group O individuals.

When anti A encounters red cells carrying A antigen, it can bind to the A determinants on those cells. Depending on the antibody concentration and reaction conditions, this interaction can produce visible agglutination and may activate complement.

The clinical importance of anti A is particularly high because strong anti A can cause rapid destruction of A antigen containing red cells when they are introduced into an incompatible circulation.

Anti B

Anti B is an antibody directed against the B antigen.

It is normally present in the plasma of individuals whose red cells do not carry B antigen.

Therefore, anti B is normally found in the plasma of group A and group O individuals.

When anti B encounters B antigen on red cells, it binds to the antigen and can produce agglutination. Strong anti B can also activate complement and cause haemolysis.

The reactions of anti A and anti B form the basis of the reciprocal antibody relationship within the ABO system.

Anti A,B

Plasma from group O individuals contains strong anti A and anti B activity. It may also contain antibody activity referred to as anti A,B.

Anti A,B is capable of reacting with red cells expressing A or B determinants and is particularly relevant when considering the antibody content of group O plasma.

It is important not to think of anti A,B simply as an equal mixture of anti A and anti B. The term refers to antibody activity that recognizes A and B related determinants.

The presence of these antibodies explains why group O plasma can be clinically important when transfused into recipients with A or B antigens.

Development of ABO Antibodies

ABO antibodies develop during the early period of life as the immune system encounters environmental antigens.

Many microorganisms and environmental substances contain carbohydrate structures that are sufficiently similar to A or B determinants to stimulate antibodies that can cross react with human ABO antigens.

For example, an individual who lacks A antigen may encounter environmental carbohydrate structures that resemble A antigen. The immune response generated against those structures can include antibodies capable of recognizing A antigen on red cells.

This explains why ABO antibodies can be present in individuals who have never received a blood transfusion.

ABO antibodies are therefore different from many immune antibodies that develop only after exposure to a corresponding red cell antigen through transfusion or pregnancy.

At birth, ABO antibody production is not fully developed. The concentration of the individual's own ABO antibodies increases during infancy as environmental exposure continues and the immune system matures.

ABO antibodies are predominantly IgM, although both IgM and IgG may occur.

IgM is particularly important because of its molecular structure. IgM is generally present as a pentamer in plasma, giving it multiple antigen binding sites. This allows one IgM molecule to interact with antigenic determinants on more than one red cell.

This property makes IgM highly effective at producing direct agglutination.

IgM is also a strong activator of the classical complement pathway. When IgM binds appropriately to antigens on a red cell surface, it can initiate complement activation and contribute to membrane destruction.

IgG ABO antibodies are also important. IgG is a smaller monomeric immunoglobulin and does not usually produce direct agglutination as efficiently as IgM in saline conditions. However, IgG can bind to red cells and can participate in their destruction through Fc receptor mediated mechanisms.

The presence of IgG is especially important in pregnancy because IgG can cross the placenta.

Why IgM Produces Strong Agglutination

Agglutination occurs when antibodies bind to antigens on separate red cells and effectively link the cells together.

The pentameric structure of IgM makes this particularly efficient.

When IgM binds to ABO antigens on one red cell, its other antigen binding sites can interact with antigens on neighbouring cells. Repeated antibody antigen interactions create a network of red cells that becomes visible as clumping.

The reaction can be represented simply as:

ABO antigen + corresponding antibody → antibody binding → red cell bridging → agglutination

The strength of agglutination depends on the number of available antigenic sites, antibody concentration, antibody affinity and the conditions under which the reaction occurs.

This explains why ABO antibodies are highly effective reagents in routine blood group serology.

ABO Antibodies and Complement

The ability of ABO antibodies to activate complement is one of the most important reasons they are clinically dangerous.

When an ABO antibody binds to its corresponding antigen on an incompatible red cell, the antibody can initiate the classical complement pathway.

Complement activation results in the sequential activation of complement proteins. Continued activation can lead to deposition of complement components on the red cell membrane and, when sufficiently extensive, formation of the membrane attack complex.

The red cell membrane can become severely damaged, resulting in release of haemoglobin directly into the plasma.

This process is known as intravascular haemolysis.

ABO antibodies therefore have two related but distinct effects that are important in laboratory and clinical settings.

They can produce agglutination, which is useful for detecting an antigen antibody reaction.

They can produce haemolysis, which is potentially life threatening when the reaction occurs inside the circulation.

Factors Affecting ABO Antibody Reactions

The strength of an ABO antibody reaction is influenced by several factors.

The concentration of antibody affects how many antigenic sites can be occupied. A higher antibody concentration can produce a stronger reaction when the corresponding antigen is present.

Antigen density also affects the reaction because a cell expressing more antigenic sites provides more opportunities for antibody binding.

Temperature is another important factor. ABO antibodies may react over a range of temperatures, but clinically significant reactions are particularly important when antibodies react under conditions approaching physiological temperature.

The physical arrangement of antigens on the red cell surface also affects the ability of antibodies to bridge neighbouring cells.

Other laboratory factors such as serum to cell ratio, incubation conditions, ionic strength and reaction time can influence the strength of the observed reaction.

Therefore, the strength of an agglutination reaction is the result of the interaction between the antibody, antigen and reaction environment rather than a simple measure of antigen presence.

An antibody titre is a measure of antibody reactivity obtained by testing serial dilutions of a serum or plasma sample.

ABO antibody titres can vary considerably between individuals.

Two people with the same ABO group may therefore have different concentrations and strengths of anti A or anti B.

Titre does not represent an absolute measurement of the total quantity of antibody in the sample. It represents the highest dilution at which the antibody still produces the defined reaction under the conditions of the test.

This distinction is important when interpreting laboratory results because titre depends on the testing method and reaction conditions.

High titre ABO antibodies are particularly important when considering transfusion of plasma containing those antibodies because the antibodies may react with ABO antigens on the recipient's red cells.

ABO Incompatibility and Acute Haemolytic Transfusion Reaction

The greatest clinical importance of ABO antibodies is their ability to cause severe haemolysis following an incompatible red cell transfusion.

Consider a situation in which red cells carrying A antigen are introduced into the circulation of a person who has strong anti A.

The anti A binds to the A antigen on the transfused red cells.

Because ABO antibodies can be present at relatively high concentrations and many are IgM, the reaction can occur rapidly.

Complement may then be activated, leading to destruction of the incompatible red cells within the circulation.

The result is an acute haemolytic transfusion reaction.

Clinical manifestations can include fever, chills, chest or back pain, hypotension, haemoglobinuria, haemoglobinaemia and, in severe cases, acute kidney injury, disseminated intravascular coagulation, shock and death.

The severity of an ABO incompatible reaction depends on several factors, including the amount of incompatible blood introduced and the strength of the recipient's antibody response.

ABO incompatibility is therefore one of the most serious preventable hazards in transfusion medicine.

Mechanism of ABO Mediated Intravascular Haemolysis

The sequence of events can be understood as follows.

An incompatible red cell enters the circulation.

The corresponding ABO antibody recognizes the antigen on the red cell.

The antibody binds to the antigen.

Complement is activated.

Complement components accumulate on the red cell membrane.

Extensive complement activation damages the membrane.

Haemoglobin is released into the plasma.

The released haemoglobin can produce haemoglobinaemia and, when filtered through the kidneys, haemoglobinuria.

Severe intravascular haemolysis can contribute to renal injury, hypotension, coagulation abnormalities and shock.

This rapid sequence explains why ABO incompatible transfusion requires immediate clinical attention.

ABO Antibodies in Pregnancy

ABO antibodies can also have clinical significance during pregnancy.

The major concern is maternal IgG anti A or anti B crossing the placenta and reacting with corresponding ABO antigens on fetal red cells.

This situation is most commonly associated with a group O mother carrying a fetus with A or B antigen.

The reason group O mothers are particularly important is that their anti A and anti B can contain a significant IgG component.

IgM does not normally cross the placenta because of its large molecular size. IgG, however, can cross the placenta and enter the fetal circulation.

If maternal IgG anti A or anti B binds to fetal red cells, it can promote their destruction.

This can result in ABO haemolytic disease of the fetus and newborn.

ABO haemolytic disease is usually less severe than severe Rh mediated haemolytic disease. One reason is that ABO antigen expression on fetal red cells is relatively limited compared with adult red cells. ABO antigens are also expressed on other tissues, which may reduce the amount of antibody available to react specifically with fetal red cells.

Most cases are mild, although clinically significant haemolysis can occur.

Haemolytic Disease of the Fetus and Newborn

In ABO haemolytic disease, maternal IgG antibody crosses the placenta and attaches to fetal red cells carrying the corresponding ABO antigen.

The antibody coated red cells may then be removed from circulation through immune mediated mechanisms.

The newborn may develop anaemia and increased bilirubin production as a result of red cell destruction.

The clinical presentation is often dominated by neonatal jaundice and varying degrees of anaemia.

ABO haemolytic disease is generally milder than Rh mediated disease and may not require treatment in many cases. Nevertheless, significant cases can occur and must be recognized.

The important immunological principle is that the antibody responsible must be an IgG antibody capable of crossing the placenta. The predominantly IgM nature of typical ABO antibodies is therefore one reason most ABO incompatibility does not produce severe fetal disease.

Why ABO Antibodies Are Clinically Important

ABO antibodies combine several properties that make them particularly significant.

They are commonly present naturally.

They can occur in relatively high concentrations.

They recognize carbohydrate antigens that are strongly expressed on red cells.

Many are IgM.

IgM is highly effective at agglutination.

IgM is also a powerful activator of complement.

Together, these characteristics allow ABO antibodies to produce a rapid antigen antibody reaction and, in an incompatible transfusion, potentially extensive intravascular haemolysis.

This is why ABO compatibility must be established carefully before administration of red cell containing blood products.

The easiest way to understand the system is to keep the antigen and antibody concepts separate.

The antigen is the carbohydrate structure on the red cell.

The antibody is the immunoglobulin in plasma that recognizes a specific antigenic determinant.

A antigen is therefore not the same as anti A.

B antigen is not the same as anti B.

A antigen is recognized by anti A.

B antigen is recognized by anti B.

A person normally does not have a strong antibody directed against the ABO antigen expressed on their own red cells. Instead, the plasma contains antibody against the major ABO determinant that is absent.

This reciprocal relationship is the central immunological principle of the ABO system.

ABO antigens are carbohydrate structures located on the outer surface of red blood cells and can also be expressed on other cells and tissues.

The H substance is the precursor structure used to produce A and B determinants.

A antigen is formed by adding N acetylgalactosamine to H substance.

B antigen is formed by adding galactose to H substance.

Group O red cells do not express A or B determinants but retain H substance.

ABO antibodies are mainly naturally occurring anti A and anti B antibodies.

Anti A reacts with A antigen.

Anti B reacts with B antigen.

ABO antibodies are predominantly IgM, although IgG components may also be present.

IgM is particularly effective at agglutination because of its multivalent structure.

ABO antibodies can activate complement and cause intravascular haemolysis.

ABO incompatible red cell transfusion can cause a severe acute haemolytic transfusion reaction.

IgG anti A and anti B can cross the placenta and cause ABO haemolytic disease of the fetus and newborn, which is usually less severe than Rh mediated disease.