ABO blood group

See Blood Groups appendix.

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ABO blood group .ā-(.)bē-'ō- n one of the four blood groups A, B, AB, or O comprising the ABO system

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the major human blood group system, dependent on the presence or absence of A and B antigens, which are largely glycolipids on the cell membrane. The gene for A is responsible for synthesis of N-acetyl-α-D-galactosaminyl transferase, whereas that for B is responsible for α-D-galactosyl transferase. Either A or B is created when one of these hexasaccharides is positioned by a specific transferase in 1→3 linkage to the β-D-galactose of an H-active oligosaccharide. Type O occurs when neither transferase is present or, very rarely (Bombay phenotype), when H antigen does not exist. When both transferases are present, type AB results. Differences in degree of transferase activity are determined at the same locus: weak transferase gives rise to weak antigens (A₂, A₃A_x, B₃B_x). Similar oligosaccharides, especially in bacterial cell walls, immunize persons lacking A or B so that their serum contains anti-A or anti-B activity. A and B antigens are on the mucopolysaccharides of secretors; persons with dominant genes have H-active mucoids.