Definition:

Inherited Antithrombin (AT) deficiency is an autosomal dominant disorder, that is associated with an increased risk for venous thromboembolism (VTE) and pregnancy loss. AT is a potent inactivator of thrombin and factor Xa and the major inhibitor of blood coagulation. This disease is divided into type I deficiency, in which both the functional activity and antigenic levels AT are proportionately reduced, and type II deficiency, in which normal antigen levels are found in association with low AT activity due to a dysfunctional protein. Type II deficiencies can be further subclassified into three types, depending on the location of the mutations. Type IIa is caused by mutations that affect AT's reactive site. Type IIb is characterized by an abnormality of the heparin-binding domain. Type IIc variants are a pleiotropic group of mutations near the reactive loop site.