A deficiency in which of the following factors of hemostasis, whose symptoms generally do not appear prior to age 15, would manifest with recurrent deep vein thrombosis, pulmonary embolism and cerebral vein thrombosis?

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Antithrombin III is the most important since it can also inhibit the activities of factors IXa, Xa, XIa, and XIIa. The activity of antithrombin III is potentiated in the presence of heparin by the following means: heparin binds to a specific site on antithrombin III, producing an altered conformation of the protein, and the new conformation has a higher affinity for thrombin as well as its other substrates. This effect of heparin is the basis for its clinical use as an anticoagulant. The naturally occurring heparin activator of antithrombin III is present as heparan and heparan sulfate on the surface of vessel endothelial cells. It is this feature that controls the activation of the intrinsic coagulation cascade. Thrombin activity is also inhibited by α₂-macroglobulin, heparin cofactor II, and α₁-antitrypsin. The characteristic features of antithrombin deficiency are deep vein and cerebral vein thrombosis and pulmonary embolism. The thromboembolic events associated with a deficiency in antithrombin are rarely seen in individuals younger than 15 years of age. The reason for this is suggested to be due to the high levels of α₂-macroglobulin present in childhood.

A Deficiency in Which of the Following Factors of Hemostasis