PF4 and Thrombocytopenia: VITT is characterized by thrombosis, mostly in unusual sites, and thrombocytopenia, and clinically resembles severe heparin-induced thrombocytopenia (HIT), a prothrombotic disorder caused by platelet-activating antibodies that recognize multimolecular complexes between cationic platelet factor 4 (PF4) and anionic heparin [71].