The present invention relates to antibodies or fragments thereof that specifically bind Fc.gamma.RIIB, particularly human Fc.gamma.RIIB, with greater affinity than said antibodies or fragments thereof bind Fc.gamma.RIIA, particularly human Fc.gamma.RIIA. The invention provides methods of enhancing the therapeutic effect of therapeutic antibodies by administering the antibodies of the invention to enhance the effector function of the therapeutic antibodies. The invention also provides methods of enhancing efficacy of a vaccine composition by administering the antibodies of the invention.