The blood group antigens are oligosaccharides expressed on the surface of red blood cells and are determined by the terminal saccharides. The biosynthesis is effected via the action of the A or B glycosyltransferase on the H antigen, to form the A or B glycan. The H antigen remains unmodified in individuals lacking either of these glycosyltransferases, resulting in an individual with blood type O. The expression of these antigens is not limited to red blood cells and also occurs on epithelial and other cells.
Blood groups are determined by the ABO gene, which encodes the glycosyltransferase which forms the A, B and H antigens. The alleles for the A and B blood types show co-dominance, whilst the blood group O allele is recessive, giving rise to the familiar 4 ABO blood types.