RF are immunoglobulins of any isotype with antibody activity directed against antigenic sites on the Fc region of human or animal IgG. RFs have been associated with three major immunoglobulin classes, IgM, IgG, and IgA. IgM and IgG RFs are the most common. Immunoglobulin M rheumatoid factor is manifested in approximately 70% of adults but is not specific for RA.