New roles for rheumatoid factor.

In the 1940s, anti-IgG autoantibodies were discovered in the sera of patients with rheumatoid arthritis and given the term rheumatoid factors (RF).' The measurement of agglutinating IgM-RF is still the most useful serological test for the diagnosis of rheumatoid arthritis. High titers of IgM-RF are also present in primary Sjogren's syndrome, mixed cryoglobulinemia, and in several different chronic infections (1).