Complex of IgG and anti-igG formed in joints in rheumatoid arthritis. Serum rheumatoid factors are more usually formed from IgM antibodies directed against IgG. This entry appears with permission from the Dictionary of Cell and Molecular Biology. Rheumatoid Factor (RF) is the autoantibody (antibody directed against an organism's own tissues) that was first found in rheumatoid arthritis. It is defined as an antibody against the Fc portion of IgG. RF and IgG join to form immune complexes that contribute to the disease process. Rheumatoid factor can also be a cryoglobulin (antibody that precipitates on cooling of a blood sample); it can be either type 2 (monoclonal IgM to polyclonal IgG) or type 3 (polyclonal IgM to polyclonal IgG) cryoglobulin. Although predominantly encountered as IgM, Rheumatoid factor can be of any isotype of immunoglobulins, i.e. IgA, IgG, IgM, IgE, IgD. An OSH glossary used in safety and health at work which is, adopted by ILO