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NE OF THE MAJOR QUESTIONS in modern immunology is how thymusderived (T) cells and bone marrow-derived (B) cells interact to achieve mutual cooperation and negative feedback control. 1 It is becoming clear that disturbances in the intricate balance between T and B cells may result in a variety of clinical disorders, especially those considered to be autoimmune syndromes. 2 This development is largely due to the fact that in experimental animal models and in the clinical presentation of these disorders, several immunologic abnormalities leading to tissue injury have been well documented and shown to be a prerequisite for the manifestation of the disease state. This review will briefly summarize the abnormal interactions between T and B cells and the way in which their derangement may lead to autoimmunity. Special attention will be placed on the enumeration of peripheral blood lymphocytes by membrane-associated cell markers and the possible pathogenetic, diagnostic, and prognostic significance of these findings. DEFINITIONS AND NOMENCLATURE For the purpose of this review, T cells will be defined as thymus-derived lymphocytes capable of initiating delayed hypersensitivity and allograft reactions, elaborating lymphokines, and participating in the defense of the host against in