Structural Components: Antigens and Antibodies
The foundation of the physiological doctrine of blood groups is based on the interaction of two key components. Their unique combination determines the individual blood profile of each specific person.
Agglutinogens (Antigens) Chemically, agglutinogens are specific carbohydrate components of molecules belonging to the glycolipid class. Their primary localization is the outer surface of the erythrocyte membrane. Notably, these antigens are not exclusive to red blood cells; they are also found on the membranes of other formed elements, such as leukocytes and platelets. Within the AB0 system, two basic antigens are distinguished: antigen A and antigen B. The complete absence of both is considered separately. An erythrocyte may contain only antigen A, only antigen B, bear both antigens simultaneously, or contain neither.
Agglutinins (Antibodies) Agglutinins are located exclusively in the liquid portion—blood plasma. By nature, they are immune system proteins, specifically class M immunoglobulins (IgM). A key rule of this system is that agglutinins possess strict specificity: they are produced exclusively against the antigen that is absent from the person's own erythrocyte membrane. An interesting physiological feature is that agglutinins are completely absent in the blood plasma of newborns; their active synthesis and accumulation occur gradually throughout the first year of life.
AB0 System Classification
Modern medical science recognizes over 300 antigen-antibody systems, but only two hold substantial clinical and practical significance: the AB0 system and the Rhesus system. Blood classification according to the expression of AB0 agglutinogens involves four main groups:
- Group I (0). Characterized by erythrocytes completely lacking agglutinogens (neither A nor B). Both types of antibodies—$\alpha$ and $\beta$ agglutinins—circulate in the plasma.
- Group II (A). Only agglutinogen A is present on the erythrocyte membrane. $\beta$-agglutinin is found in the plasma since antigen B is absent from the patient's own cells.
- Group III (B). Erythrocytes bear only agglutinogen B. $\alpha$-agglutinins are present compensatorily in the plasma.
- Group IV (AB). Erythrocytes express both agglutinogens simultaneously (both A and B). The blood plasma contains no agglutinins at all.
The physiological norm dictates that homonymous antigens and their corresponding antibodies never coexist in the blood of a healthy individual. This is a genetically determined protective mechanism preventing the destruction of one's own blood.
Mechanism of the Agglutination Reaction
The primary danger of mismatched blood transfusion is the development of an agglutination reaction. This pathological mechanism is inevitably triggered if homonymous components interact within the bloodstream.
The interaction of an antibody with its corresponding antigen (e.g., agglutinogen A meeting $\alpha$-agglutinin, or agglutinogen B meeting $\beta$-agglutinin) results in agglutination, meaning the clumping of erythrocytes together. The formation of such massive red blood cell clusters blocks normal blood flow and disrupts blood functions.
| Blood Group | Agglutinogens on Erythrocytes | Agglutinins in Plasma |
|---|---|---|
| I (0) | None | $\alpha$, $\beta$ |
| II (A) | A | $\beta$ |
| III (B) | B | $\alpha$ |
| IV (AB) | A, B | None |