AbstractRheumatoid Arthritis is a chronic, usually progressive inflammatory disorder of joints in which the immune system plays a central role in the pathogenesis. In its classic form, the synovial tissues from severely affected joints are densely infiltrated with HLA‐DR bearing T‐lymphocytes (primarily OKT4+/Leu3+subset) and macrophage‐like cells. Moreover, these tissues, as demonstrated by ex vivo culture, spontaneously produce high levels of a multitude of inflammatory mediators, such as collagenase, PGE2, interleukin 1 and fibroblast activating factors, indicating that the cells infiltrating the synovium are “activated”. The action of these various inflammatory mediators on different target substances or cells (collagen, fibroblasts, chondrocytes, osteoclasts, etc.) most likely produce the characteristic pattern of joint pathology. Recent data indicate that this classic form of synovitis tends to be associated with peripheral anergy and other qualitative and quantitative abnormalities in the peripheral blood mononuclear cells. Repeated leukapheresis can induce substantial, although transient, clinical improvement in patients with these classic features, probably as a consequence of disrupting T‐lymphocyte traffic.Rheumatoid synovitis, however, is highly heterogeneous, but can be categorized into subsets. For example, a subset of patients with highly active clinical rheumatoid arthritis exists which do not exhibit the classic features of disease. Synovial tissues from this patient subset are sparsely infiltrated by T‐lymphocytes but contain mainly macrophages and fibroblasts, as well as prominent lining layer fibrin deposition. Patients with these synovial features do not appear to respond therapeutically to repeated leukapheresis. In view of the wide immunopathologic spectrum in rheumatoid arthritis, assessment of the role, if any, of leukapheresis in therapy remains unclear, but the technique does provide a powerful investigative tool.Rheumatoid arthritis is a chronic, usually progressive, destructive disorder of joints in which effector mechanisms of the immune system appear to play a major role in its pathogenesis. Various types of apheresis have the potential to modulate immune effector mechanisms and, in theory, could have a therapeutic effect in this disorder. In this report, we present an overview of evolving concepts of the immunopathogenesis of rheumatoid arthritis and relate these concepts to our experiences with leukapheresis.