Blood Group Antigens and Antibodies: An Immunogenetic Overview

A key genetic principle here is codominance. In a heterozygote (an individual with one A allele and one B allele), both antigens are fully and simultaneously expressed, resulting in the AB blood type.

The Rh Blood Group System

The Rh system is highly complex, involving the RHD and RHCE genes which encode Rh proteins. While there are many antigens in this system, the RhD antigen is the most clinically significant.

• RhD Positive: The D antigen is present on the RBC surface.
• RhD Negative: The D antigen is absent.

Because the D antigen is highly immunogenic, RhD-negative individuals can develop Anti-D antibodies if exposed to RhD-positive blood. This is a major concern in pregnancy, as it can lead to haemolytic disease of the newborn, and in transfusions, where it can cause severe reactions.

Antigen-Antibody Reactions and Clinical Implications

When an antibody meets its corresponding antigen, a reaction occurs. This often happens in two stages:

1. Sensitisation: The antibody binds to the red cell antigen, coating the cell.
2. Lattice formation (Agglutination): The antibodies bridge multiple red cells together, causing them to clump or "agglutinate".

Large antibodies, such as IgM, have many binding sites and can cause agglutination directly. Smaller IgG antibodies often only coat (sensitise) the cells.

Haemolytic Transfusion Reactions

If incompatible RBCs are transfused (for example, giving Group B blood to a Group A recipient), the recipient's anti-B antibodies will bind to the donor's B antigens. This triggers a haemolytic transfusion reaction, where the donor cells are destroyed.

The Complement System

The complement system is a "complementary" effector arm of the immune system, consisting of plasma proteins (C1 through C9) that circulate in an inactive state. When antibodies bind to a target cell, they trigger a cascade of protein cleavage that leads to cell destruction.

There are three primary pathways of activation:

• Classical Pathway: Triggered by antibody-antigen complexes.
• Alternative Pathway: Always "at work" or constantly active at low levels.
• Lectin Binding Pathway: Triggered by the binding of lectins to carbohydrates on the pathogen surface.

Regardless of the pathway, the cascade culminates in the formation of the Membrane Attack Complex (MAC). The MAC acts like a molecular drill, punching holes in the target cell membrane, causing the cell to burst (intravascular haemolysis).

Beyond ABO and Rh: Rare Phenotypes and Clinical Conditions

While ABO and Rh are the most famous, there are 48 recognized blood group systems and hundreds of antigens. These include the MNS, Kell, Duffy, and Kidd systems.

Rare Blood Types

Some phenotypes are incredibly rare. The Rhnull blood type, often called "golden blood," lacks almost all Rh antigens and is estimated to occur in fewer than 50 individuals worldwide.

Structural Deficiencies

Not all blood group issues are immunological; some are structural. For example, deficiencies in certain structural proteins can lead to:

• Stomatocytosis: Red blood cells that appear "mouth-like" or slit-like, which reduces their elasticity and integrity.
• Chronic Haemolytic Anemia: A condition where fragile RBCs break down rapidly, leading to fatigue, jaundice, and an enlarged spleen (splenomegaly).

Summary Table: Factors Affecting Reactions

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References: Blood group antigen & antibodies_MFRS 20262027_.pdf