PF4 and major depressive disorder: Heightened platelet reactivity [5, 6] and excessive stickiness of endothelial cells to platelets [7] are important potential mechanisms for this excessive CV risk, as supported by studies reporting elevated plasma levels of platelet and endothelial activation markers (i.e., beta-thromboglobulin [β-TG], platelet factor 4 [PF4], P-selectin and E-selectin) [8–17] and increased platelet aggregability in individuals with CKD [5], as well as in those with MDD [4, 14, 18].