Immune complexes and complement in rheumatoid arthritis.

Immune complexes have been shown to occur frequently during rheumatoid arthritis. They have been found in blood, in the synovium and in other extravascular lesions. The recent development of methods for the quantitation of immune complexes provided new tools to evaluate the possible role of immune complexes in rheumatoid arthritis. Immune complexes which appear in synovial fluid are in higher concentration than in serum and have particular physicochemical properties. They likely result from a local formation in the synovium and seem to be directly involved in the generation of the local inflammation. High levels of circulating immune complexes are usually associated with the development of extra-articular vascular lesions. One of the major biological activity of immune complexes is to activate the complement system. There is indeed evidence of complement activation in circulating blood as well as in synovial fluid in patients with rheumatoid arthritis. The presence and the concentration of complement breakdown products in these fluids correlates with the clinical activity. Therefore, the analysis of immune complexes and of complement components appears useful for diagnosis and follow-up, and for the understanding of the pathogenesis of the disease.