Extracellular aggregates of immunoglobulin in synovial fluid from rheumatoid arthritis.

Particulate extracellular aggregates of immunoglobulin can be demonstrated in many whole synovial fluids from patients with rheumatoid arthritis but not in other diseases. These occur only in seropositive patients and correlate with more active inflammation and lowered synovial fluid C3. Aggregates were associated with intracellular inclusions but were more abundant and always contained both IgG and IgM. No aggregates were seen in 50 control fluids. These aggregates are compatible with immune complexes and should be included in studies of immunologic events in rheumatoid fluids.