Heparin-induced thrombocytopenia (HIT) is a serious immune-mediated drug reaction caused by heparin, a commonly used anticoagulant medication. HIT is characterized by a decrease in platelet count and an increased risk of developing blood clots, which can lead to serious complications such as stroke, heart attack, or pulmonary embolism. Diagnosis of HIT is based on clinical symptoms, laboratory tests, and the presence of antibodies against heparin-platelet factor 4 complexes. Treatment typically involves discontinuing heparin and using alternative anticoagulants to prevent further clot formation and manage the thrombocytopenia.