Rheumatoid arthritis (RA) is a chronic inflammatory disease affecting primarily the joints and periarticular tissues. For many years it has been known that several abnormal proteins circulate in the blood of patients with RA. These proteins, because of their obvious correlation with the disease, became known as rheumatoid factor (RF). Research of these proteins has characterized them as usually being a group of IgM class immunoglobulins that interact with antigenic determinants on human IgG molecules (i.e., they are anti-antibodies). Rheumatoid factor is detected in 60-80% of cases of diagnosed rheumatoid arteritis.