The present invention relates to molecules, preferably soluble (i.e., not membrane bound) polypeptides, most preferably soluble fusion polypeptides comprising the extracellular soluble regions of Fc.gamma.RIIB, derivatives and analogs thereof, and nucleic acids encoding same. Molecules of the invention are particularly useful for the treatment, management, or prevention of, or amelioration of one or more symptoms of, an autoimmune disease, especially for ameliorating serum platelet deficiency associated with immune thrombocytopenic purpura. The invention provides methods and compositions for enhancing the therapeutic efficacy of standard, current or experimental therapies for an autoimmune disease by administering a molecule of the invention.