Platelet factor 4

The gene for human PF4 is located on human chromosome 4.^[6]

Platelet factor-4 is a 70-amino acid protein that is released from the alpha-granules of activated platelets and binds with high affinity to heparin. Its major physiologic role appears to be neutralization of heparin-like molecules on the endothelial surface of blood vessels, thereby inhibiting local antithrombin activity and promoting coagulation. As a strong chemoattractant for neutrophils and fibroblasts, PF4 probably has a role in inflammation and wound repair.^[5][7]

PF4 is chemotactic for neutrophils, fibroblasts and monocytes, and interacts with a splice variant of the chemokine receptor CXCR3, known as CXCR3-B.^[8]

The heparin:PF4 complex is the antigen in heparin-induced thrombocytopenia (HIT), an idiosyncratic autoimmune reaction to the administration of the anticoagulant heparin.^[9] PF4 autoantibodies have also been found in patients with thrombosis and features resembling HIT but no prior administration of heparin.^[10] Antibodies against PF4 have been implicated in cases of thrombosis and thrombocytopenia subsequent to vaccination with the Oxford–AstraZeneca or the Janssen COVID-19 vaccine.^[11][12] This phenomenon has been termed vaccine-induced immune thrombotic thrombocytopenia (VITT).^[13] Changes in the expression of PF4 have also been associated with symptoms of long COVID.^[14]

It is increased in patients with systemic sclerosis that also have interstitial lung disease.^[15]

The human platelet factor 4 kills malaria parasites within erythrocytes by selectively lysing the parasite's digestive vacuole.^[16]

• Platelet-activating factor
• Platelet-derived growth factor