Rheumatoid arthritis primarily involves the synovial membrane of the joints, together with that of the tendon sheaths and bursae. These structures are targeted in a selective and symmetrical manner. The inflammatory cell infiltrate is usually dominated by mononuclear phagocytes. These are recruited into the synovial lining as type A synoviocytes. A proportion of the mononuclear cell population has proven to be specialized antigen-presenting accessory cells. The T and B Lymphocyte infiltrate is quite variable in intensity, but may become highly organized. Polymorphonuclear leukocytes pass rapidly into the joint space. The cellular characteristics of rheumatoid nodules have some similarities with those of the synovial membrane. In a minority of patients, immunoglobulin production and circulating immune complex formation reaches levels which precipitate the appearance of a complement mediated vasculitis. This may lead to tissue damage in remote organs.