Heparin-induced thrombocytopenia (HIT) is a serious immune-mediated adverse reaction that can occur in patients who are treated with heparin, a common anticoagulant medication. It is characterized by a significant decrease in platelet count, which can lead to potentially life-threatening blood clotting complications. HIT is caused by antibodies that recognize platelet factor 4 (PF4) bound to heparin, resulting in activation of platelets and the coagulation cascade. Diagnosis of HIT can be challenging, as it requires both clinical suspicion and laboratory testing to confirm the presence of anti-PF4/heparin antibodies. Treatment involves immediate cessation of heparin and initiation of alternative anticoagulant therapy to prevent further clotting complications.