Idiotypic and anti-idiotypic antibodies interfere in a standard radioimmunoassay for beta-endorphin.

Human anti-beta-endorphin and anti-idiotypic antibodies competitively inhibited the binding of rabbit anti-beta-endorphin antibodies to beta-endorphin. Furthermore, human and rabbit antibodies demonstrated cross-reactive idiotypic binding interactions. Competition for binding to beta-endorphin and idiotypic binding between human and commercially produced rabbit antibodies interfered in a standard radioimmunoassay to detect beta-endorphin. Purified human anti-beta-endorphin antibodies falsely elevated the calculated concentrations of samples with standardized concentrations of beta-endorphin. Anti-beta-endorphin antibodies were also falsely measured as beta-endorphin in samples that did not contain beta-endorphin. In contrast, anti-idiotypic antibodies lowered the calculated concentrations. These data suggest the occurrence of complex competition between several competitors rather than inhibition by a single competitor (i.e., unlabeled peptide) in subjects that produce specific antibody. Competitive and idiotypic binding interactions between human and animal antibodies may affect radioimmunoassay for other substances in subjects that produce specific antibodies.