Structure of ABO, H, and Lewis antigens. (A) Structure of ABO and H antigens on human red cells. H antigen formed by the action of FUT1 on oligosaccharide precursor chains in which the terminal D-galactose residue is linked to carbon 4 of the penultimate N-acetyl D-glucosamine residue (type II chain). (B) Structure of Le blood group antigens in bodily secretions. Secretor gene (FUT2) regulates the production of H antigen, which can be converted to A or B antigen if the corresponding active ABO glycosyltransferase is present. The ABH, Le^b-active structures are formed on oligosaccharide precursor chains in which the terminal D-galactose residue is linked to carbon 3 of the penultimate N-acetyl D-glucosamine residue (type I chain) If FUT 2 is deficient the Le^a active structure predominates.