Evidence for immune complexes containing antibody to the pepsin site of IgG in rheumatoid synovial fluids.

In seventy cases of rheumatoid arthritis, the synovial fluid to serum ratio for the titre of pepsin agglutinator, i.e. the antibody to the pepsin site of IgG, was studied. This was compared with the corresponding ratio for another antibody of the same immunoglobulin class and for other proteins. By this method there was evidence for local production of pepsin agglutinator in two cases and for local inhibition in two other cases. Density gradient ultracentrifugation, performed at pH 7·4 and 3·0, suggested that pepsin agglutinator in immune complexes was present in most of the synovial fluids containing this antibody. Complexes of pepsin agglutinator and F(ab') IgG, prepared , appeared to fix only small amounts of human complement. Neither did fixation of pepsin agglutinator to immune precipitates increase their complement binding activity.