A patient develops thrombocytopenia and thrombosis after heparin therapy. Platelet factor 4 (PF4)-heparin complex antibodies activate platelets via FcγRIIA receptor. Which hypersensitivity type is heparin-induced thrombocytopenia (HIT)?

• A Type II — IgG antibodies against PF4-heparin complex activating platelets and causing paradoxical thrombosis ✓
• B Type I — IgE-mediated mast cell degranulation against PF4-heparin
• C Type III — immune complex deposition of IgG-PF4-heparin in vessel walls
• D Type IV — T-cell mediated delayed hypersensitivity to heparin protein conjugates

Explanation

HIT is a prothrombotic immune disorder caused by IgG antibodies recognizing the PF4-heparin complex. These antibodies bind to FcγRIIA on platelet surfaces, causing platelet activation, aggregation, and consumption (thrombocytopenia). Activated platelets release microparticles promoting coagulation, leading to paradoxical thrombosis despite low platelet counts. This is a Type II (antibody-mediated) hypersensitivity reaction targeting a cell-surface antigen (platelet-bound PF4-heparin complex).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.