A, B antigens
Mucopolysaccharides responsible for the ABO blood group system. The A and B antigens reside on the surface of erythrocytes, and differ only in the sugar attached to the penultimate monosaccharide unit of the carbohydrate chain. This minor chemical difference makes the macromolecule differentially active antigenically. The IA, IB, and i are alleles of a gene residing on the long arm of chromosome 9 between bands 34.1 and 34.2. The IA and IB alleles encode A and B glycotransferases, and the difference in their specificities is due to differences in their amino acid sequences at only four positions. These in turn result from different missense mutations in the two alleles. The A and B transferases add N-acetyl galactosamine or galactose, respectively, to the oligosaccharide terminus. The i allele encodes a defective enzyme, so no additional monosaccharide is added to the chain. Glycoproteins with properties antigenically identical to the A, B antigens are ubiquitous, having been isolated from bacteria and plants. Every human being more than 6 months old possesses those antibodies of the A, B system that are not directed against its own blood-group antigens. These “preexisting natural” antibodies probably result from immunization by the ubiquitous antigens mentioned above. The A and B antigens also occur on the surfaces of epithelial cells, and here they may mask receptors that serve as binding sites for certain pathogenic bacteria. See Chronology, 1901, Landsteiner; 1925, Bernstein; 1990, Yamomoto et al.; blood group, Helicobacter pylori, H substance, Lewis blood group, MN blood group, null allele, oligosaccharide, P blood group, Secretor gene.