There are impressive similarities between the lymphocyte populations in lymph node and synovial membrane: similar amounts and classes of immunoglobulins are synthesized; lymphoid aggregates, when present in the rheumatoid synovial membrane, resemble those of the lymph node paracortex in being composed predominantly of helper-inducer T4 cells; B cells which leave tall endothelial PCV together with the T cells, tend to segregate themselves from the T cell aggregates and migrate to areas rich in macrophages where they proliferate and differentiate. In the lymph node, the B cells migrate to the primary follicles of the outer cortex and to the medullary region, while in the synovial membrane the corresponding region is the transitional area, which contains lymphocytes (predominantly T8 cells), macrophages and plasma cells.