A, B and H antigens on red cells are predominantly glyco-proteins, the majority being on the N-glycans of the anion exchanger (band 3) and the glucose transporter. ABH antigens on red cells are also expressed on glycosphingolipids, which include paraglobosides (Table 15.6). Soluble ABH antigens are
Table 15.6 Some glycolipids of the red cell surface expressing H, A, B, P1, PK, P and LKE activity.
Galß l ^4GlcNAcß l ^3Galß l ^4Glc-Cer Fuca l ^2Galß l ^4GlcNAcß l ^3Galß l ^4Glc-Cer Fuca l ^2Galß l ^4GlcNAcß l ^3Galß l ^4Glc-Cer 3 t l
Fuca l ^2Galß l ^4GlcNAcß l ^3Galß l ^4Glc-Cer 3 t l
Type 2 H Type 2 A
Type 2 B
Galßl^4Glc-Cer Galal^4Galßl^4Glc-Cer GalNAcß l ^3Galal ^4Galß l ^4Glc-Cer NeuAca2^3Galßl^3GalNAcßl^3Galal^4Galßl^4Glc-Cer Fucal^2Galßl^3GalNAcßl^3Galal^4Galßl^4Glc-Cer
Type 4 H
Lactosylceramide, Gb2 Globotriosylceramide, Gb3 Globoside, Gb4 Sialosylgalactosylgloboside Globo-H
Cer, ceramide; Fuc, fucose; Gal, galactose; GalNAc, N-acetylgalactosamine; Glc, glucose; GlcNAc, N-acetylglucosamine.
glycoproteins. Differences in the terminal sugars of the glycoproteins and glycolipids determine the specificity of these antigens: L-fucose (Fuc) for H; L-fucose + N-acetyl-D-galactosamine (GalNAc) for A; and L-fucose + D-galactose (Gal) for B.
Two major types of carbohydrate chain endings serve as acceptors for the fucosyltransferases that synthesize H antigen. Type 1 and type 2 chains have Gal joined to N-acetylglu-cosamine (GlcNAc) through 1^3 and 1^4 linkages respectively. A- and B-transferases synthesize the transfer of Ga1NAc and Gal, respectively, from their donor substrates UDP-GalNAc and UDP-Gal, to the terminal galactosyl residue of type IH and type 2H, creating A and B epitopes and masking H specificity (Figure 15.1).
Secretory glycoproteins possess both type 1 and type 2 linkages, whereas red cells synthesize type 2 chains only. Plasma glycolipids, passively adsorbed onto the red cells have only type 1 chains; these also carry Lewis antigens (see p. 231). Other chains, called type 3 and 4, are also present in low numbers on
FUT1 or FUT2
H-transferase (a1,2-fucosyltransferase) ' O
Figure 15.1 Biosynthetic pathway of H antigen from its precursor, and of A and B antigens from H. H remains unconverted in the absence of A or B gene products. R, remainder of molecule.
red cells, but probably only on glycolipids. Type 3, 4 and 6 chains are found in heart and kidney tissues, and may be target epitopes for the rejection of transplanted organs.
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