Binding of N-acetylgalactosamine-containing compounds by a human IgM paraprotein.

A serum that contains a monoclonal human IgM paraprotein (McG) agglutinates protease-treated human erythrocytes and binds glycosphingolipids that possess a terminal nonreducing N-acetylgalactosaminyl residue in either the alpha or beta anomeric configuration. The approximately equal reactivity of McG with both anomers of N-acetylgalactosamine was unexpected because most immunoglobulins that bind saccharides exhibit a marked preference for one anomeric configuration. The major receptor for this protein in normal group O erythrocytes is globoside, the blood group P antigen: Ga1NAc( beta, 1 leads to 3)Gal(alpha,1 leads to 4)Gal(b,1 lead to 4)G1c-ceramide. Erythrocytes of the rare Pk and p phenotypes lack globoside, and the former are not agglutinated by McG. The McG receptor in p erythrocytes, which are agglutinated as strongly as normal cells, appears to be a crossreactive glycolipid that contains a much larger number of sugar residues than globoside.