The chemical structures of the A and B antigens are shown. Two major groups: Watkins and Morgan in London, and Kabat and colleagues in New York, contributed to the chemical characterization. These antigens are not protein antigens but oligosaccharide antigens. Certain sugar residues are bound by specific linkages to form special structures in these antigens. More specifically, the immuno-dominant structure of the A antigen is specified by the presence of a fucose residue and a GalNAc residue attached to a galactose by alpha 1,2- and alpha 1,3 glycosidic linkages. The galactose residue is also linked to the internal core structure. In the B antigens, the GalNAc residue in the A antigen is replaced by a galactose residue. A structurally similar antigen was found to be abundantly present in O individuals, and was named the H antigen. The immuno-dominant structure of the H antigen lacks both the GalNAc in the A antigen and the galactose in the B antigen linked to galactose by alpha 1,3 linkage.