A method of treating a disease associated with activated B lymphocytes expressing Tetraspanin 33 (TSPAN33/BAAM). The disease can be, for example, lymphoma or an immune disease. The method includes administering an anti-TSPAN33/BAAM antibody to a patient in need of such treatment in an amount effective to treat the disease. Methods of purifying activated B cells and identifying activated and/or diseased B cells are also provided.