Heparin-induced thrombocytopenia type II (HIT II) is a prothrombotic, immune-mediated complication of heparin. What is the molecular target of pathogenic HIT antibodies, and why does thrombocytopenia paradoxically coexist with thrombosis rather than bleeding?

• A IgG antibodies target complexes of heparin with platelet factor 4 (PF4/CXCL4); these immune complexes bind FcγRIIA receptors on platelets, causing platelet activation, microparticle release, and procoagulant surface expression; simultaneously, platelet consumption in thrombi and Fc-mediated splenic destruction cause thrombocytopenia ✓
• B IgG antibodies target the heparin molecule itself and activate complement, causing platelet membrane disruption and thrombocytopenia; reduced platelet mass impairs primary hemostasis and causes bleeding
• C IgM antibodies against ADAMTS13 block VWF cleavage, causing ultra-large VWF multimers that trigger platelet aggregation without activating the coagulation cascade
• D Anti-heparin IgE antibodies trigger mast cell degranulation, releasing histamine that causes vasoconstriction and thrombosis while platelet-bound heparin is cleaved by elastase

Explanation

HIT II involves IgG antibodies (generated after 5–14 days of heparin exposure) that bind heparin-PF4 complexes. PF4 (platelet factor 4/CXCL4) is released from platelet alpha-granules and forms a highly immunogenic complex with heparin. IgG-heparin-PF4 immune complexes bind FcγRIIA (CD32a) on platelets (an activating Fc receptor), causing platelet activation, granule release, microparticle shedding, and expression of phosphatidylserine (procoagulant surface). Simultaneously, platelets are consumed in microvascular thrombi and cleared by splenic macrophages (via Fc receptors), causing thrombocytopenia. The clinical paradox is that despite thrombocytopenia, patients are at high risk for thrombosis (HITT), requiring anticoagulation with non-heparin agents (argatroban, bivalirudin).

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.