Particle counting immunoassay (PACIA). III. Automated determination of circulating immune complexes by inhibition of an agglutinating factor of mouse serum

Abstract

A factor capable of agglutinating human IgG coated particles (latex) has been found in mouse serum. This factor (MAG) was used in an unpurified form to detect circulating immune complexes in the particle counting immunoassay (PACIA) system, which allows measurement of agglutination with great precision. MAG did not react with monomeric IgG, nor with reduced and alkylated aggregated IgG. It was inhibited by immune complexes in small antigen excess. Among the various subclasses of IgG, IgA and IgM, only IgG1 and IgM when coupled to Sepharose beads displayed an inhibitory activity towards MAG. That the inhibitory factors detected in serum were immune complexes or aggregated Ig was suggested by the correlation obtained with the amounts of 'heavy' IgG found in the serum samples by Ultrogel chromatography and by the polydisperse distribution of the inhibitory factors in the heavy fraction of gradient ultracentrifugation.