What is heparin-induced thrombocytopenia?
Ordinarily, heparin prevents clotting and does not affect the platelets, components of the blood that help form blood clots. Triggered by the immune system in response to heparin, HIT causes a low platelet count (thrombocytopenia). Two distinct types of HIT can occur: nonimmune and immune-mediated. Nonimmune HIT, which occurs most frequently, is characterized by a mild decrease in the platelet count and is not harmful. The second type, immune-mediated HIT, occurs much less frequently but is dangerous. Immune-mediated HIT causes much lower platelet counts. Paradoxically, despite a very low platelet count, patients who suffer from HIT are at risk for major clotting problems. After heparin is administered to a patient, an immune complex can form between heparin and a specific blood factor (platelet factor 4, or PF4) that is released by platelets. The body views this heparin-PF4 complex as a foreign substance. Therefore, an antibody is formed against the heparin-PF4 complex. The antibody b
