Monoclonal Antibodies Against M and N Antigens

Abstract

The MN antigenic determinants reside in the amino-terminal part of glycophorin A (MN glycoprotein) of the human erythrocyte membrane (1–6). The amino-terminal part of glycophorin B (Ss glycoprotein) is structurally identical with that of glycophorin A of NN individuals (2,3,6) and therefore also carries N determinants. The N activity of MM cells is low but still large enough to cause weak reactions with some polyclonal anti-M sera particularly with red cells that also carry the S antigen since they contain more glycophorin B than ss cells (7,8). The MN blood group antigenic determinants consist of a sequence of 5 amino acids (fig 1), and acids 2–4 are substituted with identical tetrasaccharide structures (5,6). Amino acids 1 and 5 are Ser and Gly in M, and Leu and Glu in N antigen, whereas acids 2–4 are identical in both (fig 1).The sugar moiety consists of an internal Gal-Ga1NAc sequence with two molecules NeuAc attached (fig 1). Most polyclonal reagents recognize the amino-terminal end of the glycoprotein molecule including NeuAc. For this reason the majority of these sera do not react with cells that have been desialylated. Variant M and N antigens exist and the structures of some of them have been elucidated eg M^c and He (fig 1). The variant structures contain amino acid substitutions at positions 1–5 in the polypeptide chain. M^c for instance contains Ser at position 1 as normal M and Glu at position 5 as normal N.