This study involves the collection of blood samples from patients with immune thrombocytopenic purpura (ITP) to evaluate the relationship between platelet counts, blood levels of a hormone called thrombopoietin that controls platelet production by the bone marrow, and blood levels of antibodies against thrombopoietin that could interfere with the action of this hormone. Blood samples will also be stored if separately agreed to by the patient for analysis of genes that might affect platelet production. At a single outpatient clinic visit, patients will have a medical history taken, and blood samples drawn for testing. Results from this study may help further understand the control of platelet production in patients with ITP, and suggest new therapeutic approaches.