Immune complex detection and complement activity in rheumatoid arthritis: a comparative study of a radioimmunoassay using monoclonal rheumatoid factor, gel diffusion techniques and C4 activity.

Paired sera and synovial fluids from forty-nine patients with rheumatoid arthritis and twenty-five with other forms of arthritis were tested for immune complexes by a radioimmunoassay using monoclonal rheumatoid factor and gel diffusion procedures with monoclonal rheumatoid factor and C1q. Synovial fluid hemolytic C4 and C4 adjusted for IgG concentration were determined in both groups of patients. Immune complexes were detected at similar high frequencies in the rheumatoid synovial fluids by precipitin formation with monoclonal rheumatoid factor (68%) and C1q (71%). In contrast, immune complexes in rheumatoid sera were detected in low frequency by precipitin reactions with monoclonal rheumatoid factor (10%) and C1q (0%). Using the monoclonal rheumatoid factor radioimmunoassay, thirty-one (63%) synovial fluids exceeded the mean non-RA binding activity by one standard deviation. Similarly, twenty-four (49%) rheumatoid sera exceeded the mean non-RA binding activity to one standard deviation. Synovial fluid C4 adjusted for IgG as well as IgG alone distinguished between the two groups of patients whereas the C4 values did not. The C4/IgG value showed a strong negative correlation with the monoclonal rheumatoid factor radioimmunoassay and C1q precipitin formation.