A test for antigen--antibody complexes in human sera using IgM of rabbit antisera to human immunoglobulins.

A simple semi-quantitative test for soluble antigen--antibody complexes using characterized non-human reagents and permitting analysis of their constituents is described. The agglutination of immunoglobulin-coated latex particles by rabbit IgM antibodies to the immunoglobulin is inhibited by complex-containing sera. No inhibition is obtained with monomer immunoglobulins. Semi-quantitative measurement of complexes may be made by electronically counting residual unagglutinated latex particles. The new method of linking proteins to latex by DNP coupling permits the technique to be applied to all constituents of complexes; immunoglobulins, complement and antigens. A new method of decomplementing sera by EDTA-Sigma cell-IgG absorption allows analysis of sera without the false positives and false negatives other methods give. The test gave positive results for IgG complexes in most of twenty-four patients with SLE nephritis. IgA complexes were identified in a patient with Henoch-Schonlein purpura nephritis.