The specific detection of IgG, IgA and the complement components C3 and C4 in circulating immune complexes.

Four immune complex assays (PEG assays) are described; they are based on the binding of radio-labelled immunospecific antibodies to immune complexes containing IgG, IgA, C3 or C4, and subsequent precipitation with polyethylene glycol. Comparison of the results of the IgA-PEG assay with those of an existing immune complex assay (alpha-IgA-InhBA) showed that the former detects only large-sized (greater than 25 S) material. This was also demonstrated by sucrose density gradient ultracentrifugation of immune complexes in patients' sera as well as in preparations of aggregates containing IgA, C3 or C4. By using aggregates of mixed composition (IgG, IgA, C3 and C4) it was shown that each constituent could be detected by one of the 4 assays. Immune complexes containing the various constituents could also be detected in sera of patients with a variety of disorders. Further studies are needed to establish whether these constituents occur within the same complex or within different complexes.