Complement Inhibition Using Eculizumab to Overcome Platelet Transfusion Refractoriness in Patients With Severe Thrombocytopenia

Background: \- Platelets are tiny cells in the blood that help stop bleeding. Thrombocytopenia happens when people do not have enough normal platelets. Getting a transfusion of another person s platelets can help stop too much bleeding. But because these cells are from other people, the body may reject them,putting them at risk for serious bleeding complications. This conditions is called alloimmune platelet refractoriness . There are evidence that in many patients, platelet counts fail to increase after a platelet transfusion because the transfused platelets are destroyed by the body s defence soldier, called complement . Researchers want to see if a drug, that inhibits complement, can help increase platelet levels and reduce bleeding Objectives: \- To see if eculizumab increases platelet levels more after a transfusion. To see if it reduces the chance of bleeding too much. Eligibility: \- Adults 18-75 years old who have thrombocytopenia and alloimmune platelet refractoriness. Design: * Participants will be screened with medical history, physical exam, and blood tests. * Participants will have the procedures listed below. They can have them while they are in the hospital. Or they can go to the outpatient clinic for them. Each visit may take up to 3 hours. * Participants will get a meningitis vaccine if needed. Then they will get the study drug as an infusion. * Participants will have a platelet transfusion. Their blood will be drawn every 24 hours until the platelet count is less than 10,000 per 1 microliter of blood. * They will take antibiotics for 14 days. * Participants will have a checkup and blood drawn twice a week for 2 weeks. They will get more transfusions if needed.