Differential precipitation of the Clq subcomponent of the first complement component (C1) by polyethylene glycol from normal human serum and sera of patients with collagen diseases.

Sera were, under strictly standardized conditions, centrifuged in the presence of 0-5% PEG and the Clq concentration in the precipitates was measured by radial immunodiffusion. The presence of circulating immune complexes and rheumatoid factor(s) resulted in a shift of Clq precipitation to lower PEG concentrations. Clq precipitation at 1.5% PEG was shown to be specific for sera containing immune complexes. Under similar conditions addition of aggregated IgG to normal human serum gave rise to Clq precipitation directly proportional to the amount of aggregated IgG. Precipitation of endogenous Clq at 1.5% PEG and assay by radial immunodiffusion may therefore be useful for the detection and quantitation of immune complexes in human serum.