PF4 and COVID-19: Platelets from COVID-19 patients show a hyper-reactive phenotype characterized by the release of platelet factor 4 (PF4), platelet-derived growth factor (PDGF), and thromboxane A2, as well as increased P-selectin expression, which mediates platelet–leukocyte aggregate formation via P-selectin glycoprotein ligand (PSGL)-1 [6,7,8,9,10].