A 30-year-old woman develops thrombocytopenia and microangiopathic hemolytic anemia (MAHA) 5 days after starting a heparin infusion for DVT. Her platelet count drops from 220,000 to 45,000/μL. The 4T score is 6 (high probability). Which antibody-mediated mechanism underlies heparin-induced thrombocytopenia (HIT), and why does it cause thrombosis rather than simply thrombocytopenia?

• A IgG antibodies against heparin-PF4 (platelet factor 4) complexes bind the PF4-heparin antigen on platelet surface FcγRIIA receptors, causing platelet activation, platelet aggregation, and procoagulant microparticle release; simultaneously, these antibodies activate endothelial cells and monocytes, generating tissue factor-driven thrombin and a highly thrombotic state despite thrombocytopenia ✓
• B Heparin directly binds glycoprotein Ib on platelets, activating them, which triggers complement-mediated platelet lysis and thrombocytopenia; thrombosis results from complement-mediated endothelial damage
• C IgM antibodies against heparin activate the classical complement pathway on platelet surfaces, causing platelet lysis and exposing platelet phospholipids that non-specifically accelerate coagulation
• D Heparin inhibits ADAMTS13, causing vWF multimer accumulation and platelet-vWF complexes similar to TTP mechanism, producing the paradoxical thrombosis

Explanation

HIT is caused by IgG antibodies that recognize the complex of heparin with platelet factor 4 (PF4/CXCL4), a positively charged chemokine released from alpha granules. This immune complex binds to FcγRIIA (CD32a) on platelets, cross-linking these receptors and activating platelets to release further PF4, degranulate, and shed procoagulant phosphatidylserine-rich microparticles that assemble coagulation complexes. Simultaneously, the same antibodies activate endothelial cells to express tissue factor and monocytes to generate thrombin. The net result is a hypercoagulable state causing arterial and venous thrombosis — the 'HIT with thrombosis' (HITT) syndrome — paradoxically in a patient on an anticoagulant. IgG (not IgM) is the relevant antibody, and the mechanism is FcγRIIA-mediated platelet activation, not complement lysis or ADAMTS13 inhibition.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.