The present invention relates to humanized Fc.gamma.RIIB antibodies, fragments, and variants thereof that bind human Fc.gamma.RIIB with a greater affinity than said antibody binds Fc.gamma.RIIA. The invention encompasses the use of the humanized antibodies of the invention for the treatment of any disease related to loss of balance of Fc receptor mediated signaling, such as cancer, autoimmune and inflammatory disease. The invention provides methods of enhancing the therapeutic effect of therapeutic antibodies by administering the humanized antibodies of the invention to enhance the effector function of the therapeutic antibodies. The invention also provides methods of enhancing the efficacy of a vaccine composition by administering the humanized antibodies of the invention. The invention encompasses methods for treating an autoimmune disease and methods for elimination of cancer cells that express Fc.gamma.RIIB.