Tissue factor is the primary initiator of the coagulation cascade. In normal physiological conditions, following vessel damage or trauma, tissue factor (TF) forms a complex with activated factor VII (FVIIa) in the presence of calcium ions on an appropriate phospholipid membrane, and allosterically enhances the enzyme activity of this protease to catalyse the activation of factor X (FX) to FXa. Generation of FXa by the TF–FVIIa complex triggers the proteolytic conversion of prothrombin (factor II) to thrombin (activated factor II [FIIa]). This key step in the coagulation cascade is effected by the catalytic activity of the prothrombinase complex, which consists of FXa and the nonenzymatic cofactor activated factor V (FVa) in a 1:1 stoichiometric complex. The resulting burst in thrombin activity activates localised platelets and catalyses the conversion of circulating fibrinogen into an insoluble fibrin clot, thereby restoring vessel integrity.