Heparin-induced thrombocytopenia (HIT) type II is a prothrombotic disorder caused by IgG antibodies against which antigen complex, leading to paradoxical thrombosis despite low platelet count?

• A Platelet glycoprotein Ib/IX (von Willebrand factor receptor)
• B GPIIb/IIIa (fibrinogen receptor)
• C Platelet factor 4 (PF4)-heparin complex ✓
• D Phospholipid-beta-2 glycoprotein I complex

Explanation

In HIT type II, heparin binds to platelet factor 4 (PF4, a cationic platelet alpha-granule protein) forming an immunogenic PF4-heparin complex. IgG antibodies against this complex bind to FcγRIIA on platelets, causing platelet activation, aggregation, and consumption (thrombocytopenia), while simultaneously activating endothelial cells and generating thrombin — leading to a hypercoagulable, prothrombotic state. Management requires stopping heparin and using alternative anticoagulants (argatroban, fondaparinux). GPIIb/IIIa antibodies cause drug-induced thrombocytopenia; phospholipid-beta2GPI antibodies are in antiphospholipid syndrome.

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

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Written and medically reviewed by the StethoPrep medical team.