en ايمونولوژي Immunology review review <div style="text-align: justify;"><strong>Background:</strong> ChAdOx1 nCov-19 vaccine is a viral vector-based vaccine with desirable protection (about 70.4%, two weeks after the second dose). Few reports were released on thrombocytopenia associated with thrombotic events shortly after the ChAdOx1 nCov-19 vaccination. However, the exact pathophysiologic mechanism of this vaccineinduced thrombotic complication has not yet been elucidated. Vaccine-induced thrombotic thrombocytopenia syndrome (VITTS) is associated with detecting anti-platelet factor 4 (PF4) antibodies that are not yet linked to previous exposure to heparin.<br> <strong>Materials and Methods: </strong>In the current review, based on relevantly reported cases, possible mechanisms are suggested on the relationship between the anti-platelet factor 4 (anti-PF4) antibody assays, previous exposure to heparin, and the involved mechanisms of post-vaccination thrombocytopenia and thrombotic events, which might help the experts for selecting the appropriate therapeutic measures.</div> <div style="text-align: justify;"><strong>Results:</strong> Possibly involved mechanisms in VITTS after ChAdOx1 nCoV-19 vaccination include binding of anti-PF4 antibodies to heparin/PF4 complex or receptor-binding domain (RBD) protein-PF4 complex. Another mechanism could be the binding of anti-RBD antibodies to the RBD protein-PF4 complex. Finally, anti-RBD or anti-PF4 antibodies may bind to the heparin-RBD protein-PF4 complex. The binding of either of the mentioned antibodies to these complexes via the Fc/angiotensin-converting enzyme 2 receptors can cause activation/removal of platelets leading to thrombocytopenia and thrombosis.<br> <strong>Conclusion: </strong>The suggested mechanisms in this article provide a relationship between the results of anti-PF4 antibody assays, previous exposure to heparin, and the involved mechanisms of post-vaccination thrombocytopenia and thrombotic events, which might help the experts in selecting the therapeutic measures.</div> <span style="font-size:11pt"><span style="line-height:200%"><span calibri="" style="font-family:"><b><span lang="EN-CA" style="font-size:12.0pt"><span style="line-height:200%"></span></span></b></span></span></span> Anti-PF4 antibodies, ChAdOx1 nCoV-19, Heparin, Thrombocytopenia Thrombosis. 221 228 http://rmm.mazums.ac.ir/browse.php?a_code=A-10-1237-1&slc_lang=en&sid=1 Negar Mottaghi Dastgerdi mottaghi.n@iums.ac.ir 100319475328460010910 100319475328460010910 Yes Department of Pharmaceutics, School of Pharmacy, Iran University of Medical Sciences, Tehran, Iran (Email: Faghihi.h@iums.ac.ir; Homafaghihipharm@gmail.com; ORCID: 0000-0003-1505-4936) Homa Faghihi homafaghihipharm@gmail.com 100319475328460010911 100319475328460010911 No Department of Pharmacognosy and Pharmaceutical Biotechnology, School of Pharmacy, Iran University of Medical Sciences, Tehran, Iran (Email: Mottaghi.n@iums.ac.ir, Mottaghi.negar@gmail.com; ORCID: 0000-0001-8386-774X)