The present application describes antibodies that selectively bind human Fc.gamma.RIIB, with little or no binding to other human Fc.gamma.Rs, e.g., human Fc.gamma.RIIA. The invention also provides isolated bispecific antibodies comprising an antibody that selectively binds Fc.gamma.RIIB, and a second antibody that specifically binds an activating receptor. Various uses, including therapeutic uses, for those antibodies are also described, including administration with anti-tumor antibodies and methods of inhibiting immune responses and suppressing histamine release.