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 α or β 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(β,1→3)Gal(α,1→4)Gal(β,1→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.

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This work was supported by Grant AI-05336 from the United States Public Health Service.

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