PF4 and Thrombocytopenia: Vaccination-Induced Thrombotic Thrombocytopenia (VITT) is a new type of thrombocytopenia that occurs after receiving COVID-19 vaccines, and recent research has shown a mechanism similar to heparin-induced thrombocytopenia, with antibodies against the PF4–polyanion complex as the protagonist (autoimmune or atypical heparin-induced TT) [55].