Relation of cellular infiltration of rheumatoid synovial membrane to its immune response

In the rheumatoid synovial membrane, small lymphocytes leave the blood by migrating mainly through the endothelial cells of the postcapillary venule. Extravascularly they form perivascular collections composed predominantly of small lymphocytes. A large fraction of these cells undergoes transition to form perivascular collections that consist mainly of plasma cells, indicating that they were initially B‐lymphocytes. The small lymphocytes also migrate into the perivascular region where a mixture of cells is found—ie, small lymphocytes, undifferentiated blast cells, plasmablasts, plasma cells, and macrophages all in significant numbers. The presence of these cell types in close contact suggests the interaction of T‐ and B‐lymphocytes with liberation of lymphokines, which attract and immobilize macrophages and stimulate B‐lymphocytes to antibody synthesis. The findings presented suggest that both cellular and humoral immune responses occur in the rheumatoid synovium.