The use of C1q, conglutinin and low affinity rabbit IgM antibody to human Fc in a ligand coctail radioassay for detecting and characterizing immune complexes in pathological sera.

A ligand radioassay for the detection of IC which utilizes C1q, bovine conglutinin and low affinity rabbit IgM anti-human Fc in a reagent coctail, is presented. IC are first isolated from serum by precipitation in polyethylene glycol, then analysed for their ability to react with the ligand coctail. Dual-label studies with 125I and 131I-tagged ligands, designed to determine whether the ligands bound independently to IC, indicate that the binding of each ligand to IC is not significantly affected by the presence of the other two ligands. The results of assaying pathological sera for IC by the ligand coctail radioassay correlate well with the results of three other assays. The assay system is also flexible enough to allow other low affinity IgM reagents to be used which could potentially cover the whole range of immunoglobulin classes occurring in pathological IC.