Rheumatoid Factors (RFs) were described as Immunoglobulins (IgGs) in rheumatoid arthritis as detected by agglutination of rabbit IgG coated erythrocytes (1). It was later demonstrated that RFs were IgM molecules reactive with the Fc-portion of the IgG from various species. In addition to the IgM RF, subsequent studies have identified RFs of other IgG subtypes (IgG, IgA, and IgE) in sera of patients with rheumatoid arthritis, and certain other chronic diseases. Most commercial assays measure predominantly IgM RFs. It has been observed that measurement of IgG RF may be a better indicator of rheumatoid arthritis than IgM RF. IgA RF and IgE RF have been implicated in articular disease process in rheumatoid arthritis and rheumatoid vasculitis, respectively. RFs, produced by cells within the joint space, are found in joint fluids as well. RF titer, particularly IgG RF correlate with the intensity of synovitis.
RFs can be found in serum samples from a variety of autoimmune diseases including rheumatoid arthritis, Systemic lupus erythematous, Sjogren