PF4 and COVID-19: However, recent reports have found associations between COVID-19 patients and the development of heparin-induced thrombocytopenia (HIT), a condition associated with autoimmunity caused by platelet-activating anti-platelet factor 4 (PF4) antibodies forming an immune complex with heparin (Brodard et al. 2021; Nazy et al. 2021; Nguyen et al. 2017).