Schematic representation of HIT model. (Left panel) Low or normal platelet-associated PF4 levels result in low expression of HIT antigens on platelet surfaces, with resulting low potential for platelet activation in presence of HIT antibodies. (Right panel) In contrast, high platelet-associated PF4 levels lead to increased expression of HIT antigens on platelet surfaces, predisposing to greater HIT antibody-induced platelet activation and hence greater risk of HIT.