What is "rheumatoid factor"

n. autoantibody that is usually present in the serum of people with rheumatoid arthritis

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 (an antibody against an antibody). 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.