Platelet factor 4 (PF4) is a protein released by platelets during blood clotting. It plays a crucial role in the body's response to injury by helping to regulate blood coagulation and inflammation. PF4 binds to heparin, a substance that prevents blood clotting, and can influence the activity of various cells involved in the immune response. In addition to its role in clotting, PF4 is also involved in the development of certain medical conditions. Elevated levels of PF4 can be associated with heparin-induced thrombocytopenia (HIT), a serious condition where the body forms antibodies against platelets, leading to a risk of abnormal blood clotting.