Three epitopes on the human Rh antigen D recognized by 125I-labelled human monoclonal IgG antibodies.

Seven purified monoclonal antibodies specific for the D antigen of the human Rh blood group system were examined for the characteristics of their reactions with red cells (phenotype CcDEe). The average number of sites available for binding to the antibodies ranged from 8,900 to 26,000/cell. In mutual inhibition studies, all the antibodies inhibited each other but the extent of inhibition varied in that antibodies recognizing a lower number of sites only partially inhibited those recognizing a higher number of sites. It was concluded from the evidence that these six monoclonals recognize at least three different epitopes on the D peptide. In order to explain the variation in the number of epitopes on each polypeptide, it is suggested that there is heterogeneity in the placement of the molecule in the red cell membrane resulting in variation in access to the epitopes.