The binding characteristics of anti-Rho(D)-positive human and chimpanzee erythrocytes measured with 14C protein A.

The relation of the human erythrocyte Rho(D) antigen to its chimpanzee counterpart is currently unresolved. To explore this relation, we prepared 14C protein A to compare the binding parameters between human anti-Rho(D) and Rho(D)-positive human and chimpanzee erythrocytes. Double reciprocal plots of the binding in each case exhibited a straight line with intercepts indicating approximately the same numbers of Rho(D) binding sites for human and chimpanzee erythrocytes but an affinity that was 20 times greater for the human. This would indicate that the human and chimpanzee each have a single class of sites whose primary structure is very similar but sufficiently different to give the large divergence in affinity as measured by the human isoimmune antiserum.