Autoimmune-based thrombocytopenia can be a life-threatening adverse event associated with viral load, surgery, drug therapies or the use of the anticoagulant, heparin. This grant will define mechanisms of anti-platelet antibody-dependent platelet activation and assess shedding of platelet-specific glycoprotein (GP)VI as an immediate consequence of this activation, provide a new strategy for evaluating risk of thrombosis in HIT.