the present invention relates to polypeptide compositions which bind to cell surface epitopes and , in multivalent forms , cause or lead to the killing of cells including lymphoid tumor cells , and in the case of monovalent forms , cause immunosuppression or otherwise inhibit activation of lymphocytes . the invention further relates to nucleic acids encoding the polypeptides , methods for the production of the polypeptides , methods for killing cells , methods for immunosuppressing a patient , pharmaceutical , diagnostic and multivalent compositions and kits comprising the polypeptides and uses of the polypeptides .