Photopheresis: A process in which peripheral blood is exposed in an extracorporeal flow system to photoactivated 8-methoxypsoralen (METHOXSALEN) and ultraviolet light - a procedure known as PUVA THERAPY. Photopheresis is at present a standard therapy for advanced cutaneous T-cell lymphoma; it shows promise in the treatment of autoimmune diseases.PUVA Therapy: Photochemotherapy using PSORALENS as the photosensitizing agent and ultraviolet light type A (UVA).Methoxsalen: A naturally occurring furocoumarin compound found in several species of plants, including Psoralea corylifolia. It is a photoactive substance that forms DNA ADDUCTS in the presence of ultraviolet A irradiation.Dermatitis, Exfoliative: The widespread involvement of the skin by a scaly, erythematous dermatitis occurring either as a secondary or reactive process to an underlying cutaneous disorder (e.g., atopic dermatitis, psoriasis, etc.), or as a primary or idiopathic disease. It is often associated with the loss of hair and nails, hyperkeratosis of the palms and soles, and pruritus. (From Dorland, 27th ed)Extracorporeal Circulation: Diversion of blood flow through a circuit located outside the body but continuous with the bodily circulation.Sezary Syndrome: A form of cutaneous T-cell lymphoma manifested by generalized exfoliative ERYTHRODERMA; PRURITUS; peripheral lymphadenopathy, and abnormal hyperchromatic mononuclear (cerebriform) cells in the skin, LYMPH NODES, and peripheral blood (Sezary cells).Immunosuppression: Deliberate prevention or diminution of the host's immune response. It may be nonspecific as in the administration of immunosuppressive agents (drugs or radiation) or by lymphocyte depletion or may be specific as in desensitization or the simultaneous administration of antigen and immunosuppressive drugs.Lymphocyte Subsets: A classification of lymphocytes based on structurally or functionally different populations of cells.Infection: Invasion of the host organism by microorganisms that can cause pathological conditions or diseases.Graft vs Host Disease: The clinical entity characterized by anorexia, diarrhea, loss of hair, leukopenia, thrombocytopenia, growth retardation, and eventual death brought about by the GRAFT VS HOST REACTION.