A simple, rapid and sensitive assay for immune complexes using a Staphylococcus aureus immunoadsorbent.

A simple, rapid and sensitive assay for immune complexes which does not require maintenance of cell lines or purification of proteins is described. The method consists of preferential precipitation of complexed immunoglobulin followed by binding to Staphylococcus aureus and detection by radiolabelled protein A. 3-6 microgram/ml of aggregated IgG can be readily detected in the presence of normal human serum. Soluble in vitro produced complexes can be detected over a very wide range from 4 to more than 100 fold excess of antigen over equivalence. Complexes were found in sera from patients with a range of autoimmune disorders or lung cancer. Fractionation of SLE serum showed the presence of two peaks of complex material of higher molecular weight than IgG. Interference by free immunoglobulin or antibodies to S. aureus has been discounted. Evidence is put forward for the presence of two or more binding sites for protein A on the IgG molecule.