PF4 and Thrombocytopenia: However, besides cooperating in host defense from pathogens, the anti-PF4/polyanion antibodies can also induce the so-called heparin-induced thrombocytopenia (HIT) without prior heparin exposure, a response known as spontaneous HIT [86] (Figure 2B), an immune complication of heparin therapy clinically associated with thrombocytopenia and life-threatening thromboembolic complications with mortality which exceeds 20% [87].