Diagnostic specificity and possible pathogenetic significance of inclusions in synovial leucocytes

Synovial fluid leucocytes from 70 patients with rheumatoid arthritis contained cytoplasmic inclusions; homogenates of these cells gave positive tests for rheumatoid factor in 66 (94 per cent), including 15 who had no demonstrable rheumatoid factor in serum or synovial fluid. Similar inclusions were seen in 14 of 71 patients with other rheumatic diseases, but only 3 (4 per cent) gave positive tests for rheumatoid factor. The inclusions apparently consisted of phagocytized precipitates containing rheumatoid factor; in vitro phagocytosis of rheumatoid factor‐fraction II precipitates activated lysosomal enzymes.