A radioimmunoassay for human antigen-antibody complexes in clinical material.

Soluble antigen-antibody complexes have been detected by a radioimmunoassay with rheumatoid factor as the primary antibody to bind 125-I immune complexes or heat aggregated IgG. Bound complexes are precipitated by rabbit anti-human IgM. The binding of the rheumatoid factor to the iodinated altered IgG can be inhibited by preincubation of the rheumatoid factor with human antigen-antibody complexes. The degree to which immune complexes inhibit the binding of rheumatoid factor to heat aggregated IgG has been shown to be dependent upon the antigen-antibody ratio of the complexes, with complexes formed at or near the point of equivalence exhibiting maximum inhibitory activity. Sera from individuals suffering from certain connective tissue disorders were found to inhibit the binding of rheumatoid factor to 125-I aggregrated IgG; sera from normal individuals did not inhibit this reaction when used at comparable dilutions. This procedure can be used as a relatively simple, reproducible means of detecting immune complexes in clinical specimens. The assay is sufficiently sensitive to detect 125 ng of soluble immune complexes.