IgG RFs are unique antibodies since they form immune complexes by self-association without the presence of separate antigen molecules. These immune complexes can drive inflammation by complement activation and by interaction with monocytes. Plasma cells in the synovial tissues of patients with RA synthesize IgG RFs and other rheumatoid factors. The reason for the accumulation of these cells in the synovium has not been elucidated. The detection and quantitation of IgG RFs has been difficult owing to the unique nature of these antibodies. The methods for this purpose, however, have improved. In patients with RA the presence of high levels of IgG RFs in serum is associated with clinical evidence of vasculitis. The quantitation of IgG RFs in serum of patients has not yet been established as a diagnostic or prognostic tool. Available evidence, however, suggests that these antibodies have a significant role in the pathogenesis of rheumatoid arthritis.