CXCL4/PF4 is a secreted chemokine that is expressed by megakaryocytes and restrains the coagulation cascade. It is released by activated platelets and regulates Thrombin/Thrombomodulin complexes, activated Protein C (APC), Coagulation Factor Xa, and Fibrin. The anticoagulant heparin neutralizes CXCL4 procoagulant effects. Complexes of heparin and CXCL4 trigger antibody formation that causes the pathological syndrome HITT. Immunogenic complexes of CXCL4 with Apolipoprotein H contribute to antiphospholipid syndrome (APS). CXCL4 interferes with the proliferative and angiogenic functions of FGF-2 and VEGF. However, it can also exert proinflammatory and pro-atherogenic effects on monocytes, macrophages, and endothelial cells. CXCL4 may signal through CXCR3B in humans.