Rheumatoid factor has increased reactivity with IgG from synovial fluids of patients with rheumatoid arthritis and osteoarthritis.

Synovial fluid IgG may be altered in rheumatoid arthritis (RA) and promote the formation of immune complexes with rheumatoid factor. To investigate this possibility, monomeric IgG was prepared from synovial fluids from a range of arthritides for use as the antigen in a rheumatoid factor microplate radioimmunoassay. In comparisons with normal serum IgG antigen, increased rheumatoid factor binding was shown to IgG antigens prepared from synovial fluids from patients with RA and osteoarthritis (OA). Increased binding was also shown to RA sera IgG, but not to OA sera IgG. This increased binding was not due to increased IgG antigen binding to the plate or to IgG rheumatoid factor in the antigen preparations. It was considered that cause was a structural alteration of the IgG as a result of inflammation within the rheumatoid and OA joint.