Rituximab, primarily indicated for treatment of lymphoma, has become a first line agent in the treatment of post transplant lymphoma. However, it has been used with increased frequency to treat various autoimmune diseases including those such as rheumatoid arthritis that have been thought to be primarily T-cell mediated and others such as immune thrombocytopenia purpura that have been thought to be primarily B-cell mediated. In a trial being conducted by the Immune Tolerance Network, ANCA positive vasculitis is being treated with rituximab and it appears that a short course of rituximab leads to tolerance in this autoimmune disease. Rituximab's mechanism of action in the various autoimmune diseases for which has efficacy has been variously suggested to result from elimination of either circulating autoantibody, by elimination of B-cell produced cytokines, or by interference with the antigen presentation provide by B cells.