Idiotypic antigenic determinants on human monoclonal immunoglobulins were studied by hemagglutination inhibition (HAI) by using human O Rh-positive erythrocytes coated with the proteins by the chromic chloride method. The rabbit antisera to the purified proteins required initial absorption with washed normal human erythrocytes and the hemagglutinating titer of anti-idiotypic antibodies after absorption varied from 1:1,300 to 1:328,000 for different antisera. Specificities against non-idiotypic V-region determinants, such as V-region subgroups, could not be demonstrated in the antisera.
The HAI method is both sensitive, with inhibition detected at inhibitor concentrations of 0.001 mg/ml, and specific, since heterologous proteins did not produce significant inhibition. In studies with a single myeloma protein, inhibition was only obtained with the whole homologous protein and fragments thereof which contained the combining site, e.g., F (ab′)2 and Fab. HAI is a satisfactory method for studying idiotypic specificities in human monoclonal immunoglobulins and should permit analysis of sharing of specificities and should prove suitable for the detection of residual myeloma protein in the serum of patients with multiple myeloma during their response to therapy.