Immune thrombocytopenia (ITP) is an acquired blood disease in which the individual's immune system destroys platelets, the blood cells responsible for clotting. A number of standard treatments exist to decrease the destruction of platelets, including drugs such as the steroid hormone prednisone, or removal of the spleen. Over a third of adult patients will not maintain adequate platelet counts with these treatments. Alternative treatments may be indicated due to bleeding symptoms or baseline platelet counts less than 20,000/ul, a level at which spontaneous serious bleeding can occur. Therapy for chronic ITP is generally effective in less than 30-50% of patients, however, and most of these agents have significant toxicities with long-term use, are expensive, or their administration interferes with daily activities.