PF4 and major depressive disorder: Indeed, an increased responsivity to the platelet-activating stimuli (i.e., collagen, thrombin), the increased plasma level of platelet granule contents indicative of platelet activation and subsequent coagulation process (i.e., β-thromboglobulin, factor four (PF4 factors VII and X)), and the platelet aggregation have been shown in patients with the major depressive disorder as compared to healthy controls [70,71].