Morgan and Watkins (1969), and Kabat et al. (1973) established the antigenic structures responsible for the specificity of blood groups A, B, and H. The three oligosaccharides have very subtle but immunoprotein differences (Fig.1). The basic oligosaccharide unit is a tetra-saccharide which, after taking up fucose, makes the ‘H-substance’ and is responsible for the ‘O’ blood group. Attachment of an N-acetylgalactosamine to the terminal galactose in H-substance transforms it into the ‘A’ antigen. whereas substitution with a galactose makes the ‘B’ antigen (Fig. 1).
A number of RBC membrane proteins have been implicated as carriers of the blood group determinant oligosaccharide (Hakamori 1999), these include band3 and band 4.5 glycoproteins. Donald (1981) discovered a short-chain trisaccharide (Gail11C3GalNAc) linked to the transmembrane protein ‘glycophorin’, this glycoprotein could also be an ABH carrier.
Hakamori (1999) studied the red cell membrane glycosphingolipids (GSL) and reported that the ABO oligosaccharide may be composed of different types of chains depending upon the type of glycosidic linkages and branched or unbranched structure. Type 2 chains are most common among ABO antigens, whereas Type 1 chains are more common in other tissues, e.g. gastrointestinal epithelium (Fig. 2). For both A and H determinants, there are two unbranched and two-branched type 2 chain GSLs. Unbranched A and H are predominant in fetal and umbilical cord erythrocytes, whereas branched A and H are the major components in adult erythrocytes. Blood group A-active glycosphingolipids, separated by thin-layer chromatography, are named Aa, Ab, Ac or Ad according to their order of chromatographic migration. Similarly, blood group H-active glycosphingolipids, according to their order of migration on thin-layer chromatography, are named H1, H2, H3, or H4.