Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Heparin-induced thrombocytopenia ?
    ### Response:
    Heparin-induced thrombocytopenia (HIT) is an adverse reaction to the drug heparin resulting in an abnormally low amount of platelets (thrombocytopenia). HIT is usually an immune response which typically occurs 4-10 days after exposure to heparin; it can lead to serious complications and be life-threatening.
    