T cells play a key role in the pathogenesis of type IV autoimmune reactions and also are critical for generating the T-cell-dependent autoantibodies mediating type II and type III autoimmune diseases. Consequently, considerable effort has gone into the development of therapeutic agents that selectively or nonselectively target T lymphocytes. Drugs that target primarily T cells include cyclophosphamide, azathioprine, cyclosporin A, tacrolimus and the biological CTLA4-Ig. Cyclophosphamide is an alkylating agent that substitutes alkyl radicals into DNA and RNA. The drug is inactive by itself but is converted to an active metabolite responsible for its immunosuppressive effects. It is used for the treatment of lupus nephritis and other life-threathening complications of SLE and other systemic autoimmune diseases.