Factor VIII

Coagulation factor VIII (Factor VIII, FVIII, also known as anti-hemophilic factor (AHF)) is an essential blood clotting protein. In humans, it is encoded by F8 gene. Defects in this gene result in hemophilia A, an X-linked bleeding disorder.

Factor VIII is produced in the liver's sinusoidal cells and endothelial cells outside the liver throughout the body. This protein circulates in the bloodstream in an inactive form, bound to a plasma carrier (another protein) called von Willebrand factor, until an injury that damages blood vessels occurs. In response to injury, coagulation factor VIII is activated and separates from von Willebrand factor. The active protein (sometimes written as coagulation factor VIIIa) interacts (by an as-yet-unknown mechanism) with another coagulation factor called factor IX. This interaction sets off a chain of additional chemical reactions that form a blood clot.

Factor VIII participates in blood coagulation; it is a cofactor for factor IXa, which, in the presence of Ca²⁺ and phospholipids, forms a complex that converts factor X to the activated form Xa. The factor VIII gene produces two alternatively spliced transcripts. Transcript variant 1 encodes a large glycoprotein, isoform a, which circulates in plasma and associates with von Willebrand factor in a noncovalent complex. This protein undergoes multiple cleavage events. Transcript variant 2 encodes a putative small protein, isoform b, which consists primarily of the phospholipid binding domain of factor VIIIc. This binding domain is essential for coagulant activity.

People with high levels of factor VIII are at increased risk for deep vein thrombosis and pulmonary embolism. Copper is a required cofactor for factor VIII and copper deficiency is known to increase the activity of factor VIII.

Factor VIII is available as a medication that is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system.