Heparin-induced thrombocytopenia (HIT) is a severe immune-mediated reaction to heparin, characterized by thrombocytopenia and an increased risk of thrombosis. Its pathophysiology is centered around the formation of antibodies directed against platelet factor 4 (PF4)/heparin complexes. These PF4/heparin antibodies engage platelet-FcγRIIa, leading to platelet activation and subsequent degranulation, aggregation, and the release of procoagulant extracellular vesicles (EVs). Activation of neutrophils, monocytes, and endothelial cells have also been suggested to be important features of HIT; neutrophil extracellular traps (NETs) are increasingly recognized as key contributors to thrombus propagation, monocytes may stimulate a prothrombotic state via FcγRIIa-mediated generation of tissue factor and thrombin, and endothelial activation may lead to the exposure of von Willebrand factor, further enhancing platelet recruitment and thrombosis. Importantly, interactions between different cell types, directly or indirectly, for instance, via EVs or dynamic shuttling of PF4, may consequently influence HIT responses. Vaccine-induced thrombotic thrombocytopenia (VITT), also a rare but serious complication of thrombocytopenia and thrombosis reported after administration of adenoviral vector COVID-19 vaccines, shares mechanistic parallels with HIT but is initiated by antibodies directed against PF4. These VITT antibodies also activate platelets via the FcγRIIa and may also induce the release of NETs, which could contribute to thrombus formation. Overall, in both HIT and VITT there appears to be a complex antibody-mediated interplay between various cells in promoting the regulation of thromboinflammatory responses. However, critical gaps remain regarding the precise cellular interactions driving thrombosis and/or thrombocytopenia. Further research is essential for developing improved diagnostic and therapeutic strategies for these life-threatening complications.
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