ABH antigens:
The ABH antigens are carbohydrates structures that represent the ABO blood type. ABO is also known as the major blood type since transfusions that are incompatible in this system can cause acute and often life-threatening hemolytic transfusion reactions. This is due to the fact that antibodies in this system are naturally occurring and are potent activators of complement at 37ºC.
The ABO blood type was first discovered when Karl Landsteiner noted agglutination following the mixing of sera and red cells from different individuals. The presence of naturally occurring anti-A and/or anti-B antibodies in individuals lacking the cognate antigen (also known as “Landsteiner’s Law”) is one of the defining features of the ABO system and is a useful property for confirming a patient’s ABO blood type.
Within the ABO type, a foundational antigen known as H is modified by glycotransferases to have the addition of either a monosaccharide (N-acetylgalactosamine (GalNAc) for type A, or galactose for type B). Type AB red cells, which occur due to the codominant inheritance of the A and B alleles, have the presence of both GalNac and galactose. Because these glycosyl are co-dominant, inheritance of both results in blood type AB. In the absence of both glycotransferases, the H antigen is left unmodified and the patient types as O.
Subtypes of A and B also exist. 80% of type A individuals are classified as A1 type, whereas 20% lack the A1 determinant. The most common non-A1 subtype is A2, which differs quantitatively and qualitatively from A1: A2 red cells have less antigens per red cell (1-4 x 105) compared to A1 red cells (8-12 x 105). Additionally, A2 red cells lack specific glycolipids, thereby allowing for the occurrence of anti-A1 antibodies in A2 individuals. Of note, anti-A1 is found in about 1-2% of A2 and 25% of A2B individuals.
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