An important factor in blood transfusion is the ABO blood group system, which essentially involves three antigens called A, B, and O (Table F10.1.1). These antigens are glycoproteins and glycolipids and differ only slightly in their composition. They are present on the surface of erythrocytes and are attached to the extracellular domains of integral membrane proteins called glycophorins. The presence of A, B, or O antigens determines the four primary blood groups: A, B, AB, and O. All humans have enzymes that catalyze the synthesis of the O antigen. Individuals with A blood group have an additional enzyme (N-acetylgalactosamine transferase or Aglycosyltransferase) that adds N-acetylgalactosamine to the O antigen. Individuals with B blood group have an enzyme (galactose transferase or B-glycosyltransferase) that adds galactose to the O antigen (Fig. F10.1.1). Individuals with the AB blood group express both enzymes, whereas individuals with type O blood group lack both enzymes. In humans, ABO genes consist of at least seven exons, and they are located on chromosome 9. The O allele is recessive, whereas A and B alleles are codominant.
The differences in the carbohydrate molecules of these antigens are detected by specific antibodies against either A or B antigens. Individuals with A antigens possess serum anti-B antibodies that are directed against the B antigen. Individuals with B antigens possess serum anti-A antibodies that are directed against A antigen. Individuals with blood group AB do not have antibodies directed against A or B antigens. Thus, they are universal acceptors of any blood type. Group O individuals have both anti-A and anti-B antibodies in their serum and neither A nor B antigens on their erythrocytes. Thus, these individuals are universal blood donors.
If an individual is transfused with blood of an incompatible type, the recipient’s antibodies will attack the donor erythrocytes, causing a hemolytic transfusion reaction, or destruction of the transfused erythrocytes. To prevent such be always cross-matched to the blood of a recipient. In this procedure, serum from the recipient is tested against the donor’s erythrocytes. If there is no reaction to this cross-match test, then the donors’ blood can be used for the transfusion.
Rh blood group system The other important blood group system, the Rh system, is based on the Rhesus (Rh) antigen. In humans, this system is represented by a 40-kilodalton transmembrane nonglycosylated Rh30 polypeptide that shares antigenic sites with rhesus monkey erythrocytes. Rh30 polypeptide is a component of a larger (90-kilodalton) erythrocyte integral membrane protein complex that includes Rh50 glycoprotein. Although the Rh30 polypeptide expresses many antigen sites on its extracellular domain, only three of them—D, C, and E antigens—have clinical significance. Interactions between Rh30 and Rh50 molecules are essential for the expression of D, C, and E antigens. An individual who possesses only one of these three antigens is referred to as Rh positive (Rh). All three antigens stimulate production of anti-Rh antibodies in individuals without the same antigens.