BACKGROUND: Heparin-induced thrombocytopenia (HIT) is a serious adverse reaction to heparin, associated with increased risk of thromboembolic complications. Intravenous immunoglobulins (IVIG) have been used as a therapeutic for HIT and are believed to alleviate thrombocytopenia and reduce thrombosis risk. Yet the anti-thrombotic effects of IVIG in HIT remain underexplored. OBJECTIVE: To investigate the effect of IVIG on thrombus formation in an ex vivo model of HIT-IgG-induced thrombosis. METHODS: Microfluidic channels were coated with a confluent monolayer of human umbilical vein endothelial cells (HUVECs) that were primed with TNF-α to induce an activated, inflammatory state. Whole blood was exposed to unfractionated heparin, with or without IVIG before subjecting to treatment with a monoclonal HIT-like antibody (K070), or HIT-patient-IgG. Recalcified blood was perfused over HUVECs at a venous shear stress. Thrombus structure and dynamics were investigated by immunofluorescence microscopy. RESULTS: HIT-patient-IgGs and K070 induced thrombus formation in the presence of prophylactic heparin exposure, over TNF-α-treated, inflamed endothelial cells. HIT thrombi were enriched in fibrin, phosphatidylserine-bearing platelets, and leukocyte aggregates. We observed thrombi being formed on adherent platelets, which gradually recruited leukocytes into a three-dimensional thrombus structure. Pre-treatment of blood with IVIG significantly reduced cellular adhesions and prevented thrombus formation. CONCLUSIONS: Our endothelialized ex vivo flow chamber system effectively recapitulates the immunothrombotic phenotype of HIT and offers a reliable tool to urgently validate the efficacy of IVIG intervention against HIT-IgG-induced thrombosis in patients.