Antitrombiini

Antitrombiini, commonly known as antithrombin (often antithrombin III), is a natural serine protease inhibitor that regulates blood coagulation. It primarily inhibits thrombin (factor IIa) and factor Xa, and to a lesser extent other activated coagulation factors. Antitrombiini is produced mainly by the liver and circulates in plasma as a single-chain glycoprotein that helps keep thrombin activity in check.

The inhibitory activity of antitrombiini is greatly enhanced by binding to heparin or heparan sulfate. Heparin acts as a catalyst, accelerating the formation of a stable complex between antitrombiini and its target proteases. Unfractionated heparin accelerates inhibition of thrombin and factor Xa, whereas low molecular weight heparins mainly enhance inhibition of factor Xa. This mechanism makes antitrombiini a central component of the body’s endogenous anticoagulant system and a key partner in heparin-based therapies.

Clinical relevance includes inherited (congenital) antitrombiini deficiency, which increases the risk of venous thrombosis, often in combination with other risk factors. Acquired deficiencies can occur in liver disease, nephrotic syndrome, DIC, or during severe illness. Deficiency is diagnosed by measuring antitrombiini activity and antigen levels; discordant results help distinguish type I from type II deficiencies.

Therapeutically, antitrombiini concentrates derived from human plasma or recombinant forms are used to replace deficient or dysfunctional antitrombiini, particularly in patients with heparin resistance, before surgeries, or in certain thrombotic disorders. In many settings, treatment decisions depend on AT activity levels, the risk of thrombosis, and the anticipated need for heparin anticoagulation.