We have measured the binding of anti-H-2K monoclonal antibodies to purified erythrocytes of a number of mouse strains. The purified erythrocytes specifically bound between 500 and 2000 molecules of labeled anti-H-2K antibodies per cell, whereas lymphocytes bound approximately 10(5) molecules of antibody per cell. Not all of the antibodies that bound to lymphocytes bound to erythrocytes. Antibodies to both public and private specificities that reacted with H-2k lymphocytes failed to react with H-2k erythrocytes. A similar pattern was found when comparing H-2b erythrocytes with lymphocytes. Failure to bind erythrocytes is not a function of antibody isotype or avidity. Isolated H-2K antigens of erythrocytes are integral membrane proteins identical in apparent m.w. and very similar in charge heterogeneity to the H-2K antigens of lymphocytes. Limited peptide maps of exogenously labeled antigens are also nearly identical. We also find that antibodies against epitopes of all three external domains of H-2K may react with erythrocyte H-2K. The altered reactivity of erythrocyte H-2K antigens is unlikely to be due to domain deletion, or to differences in membrane environments in erythrocytes and lymphocytes. The difference in reactivity could be due to subtle post-transcriptional modifications, or to expression of other class I genes in erythroid precursors.

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