The detection of circulating immune complexes containing immunoglobulin G.

An immune complex assay using radiolabelled immunospecific antibodies against human IgG and polyethylene glycol precipitation (IgG-PEG assay) is described. The material reactive in this assay was evaluated using aggregated immunoglobulins, immune complexes prepared in vitro, sera of patients with a variety of disorders and normal human serum. Sucrose density gradient ultracentrifugation showed that only large-sized immune complexes (greater than 25 S) were detected. Comparison of the results of the IgG-PEG assay with those of the C1q binding assay showed a highly significant positive correlation (p less than 0.001). It was found that rheumatoid factors do not influence the results of the IgG-PEG assay. The method described in this study detects specifically immune complexes containing IgG and might be extended to the detection of other constituents of circulating immune complexes.