A method for the isolation of circulating immune complexes by co-precipitation with an equine Clq-like factor is described. The Clq-like factor co-precipitated 80% of the 125I-labeled aggregated human IgG (AHG) following dialysis against 0.05 M Tris-HCl, pH 8.1 compared to only 2% of the unaggregated human IgG (HG). When incubated with bovine serum albumin (BSA) anti-BSA soluble immune complexes (IC) prepared at antigen (Ag)-antibody (Ab) equivalence or in Ab excess, the Clq-like factor co-precipitated 100Z of the available complexes. Under conditions of 2x Ag excess, Clq-like factor was less efficient as demonstrated by co-precipitation of 40% of the available IC. Although the nature and specificity of binding of the Clq-like factor is unknown, its isolation and partial characterization are reported.
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