Quantitative determination of circulating immune complexes by inhibition of the hemolytic activity of polyclonal IgM rheumatoid factor.

A practical and sensitive method for detection and quantification of soluble complement-fixing immune complexes in sera of patients with various disease states has been developed. The assay is based on inhibition of complement-dependent sheep red cell hemolysis mediated by polyclonal IgM rheumatoid factor. Aggregated human IgG was used as an in vitro model of C-fixing human immune complexes and was quantified by its ability to inhibit hemolysis of sensitized sheep red cells by isolated IgM rheumatoid factor. The limit of sensitivity of this assay was 1--3 micrograms/ml. Fixation of complement in competition with isolated IgM rheumatoid factor, resulting in inhibition of hemolysis of sensitized sheep red cells, was used for detection and quantification of immune complexes in human sera. IgM rheumatoid factor was incubated with sensitised sheep red cells followed by addition of test sera; guinea pig complement was added; and the amount of IgM rheumatoid factor mediated hemolysis was determined spectrophotometrically and referred to a standard curve of inhibition of hemolysis by increasing amounts of aggregated human gamma-globulin. Good discrimination between sero-positive rheumatoid arthritis and systemic lupus erythematosus patients compared with normal and hospitalized subjects was found.