PF4 and Thrombocytopenia: The pathogenesis of HIT has been recently better defined: during the early phase heparin binds to platelet factor 4 (PF4), generating a complex (heparin-PF4) toward which IgG antibodies are produced and this complex is also able to activate platelets via FcγRIIa receptor, causing microthrombosis and thrombocytopenia.