Which characteristic describes antithrombin (AT)?

Antithrombin (AT) is a critical component of the body's coagulation system, and one of its defining characteristics is its function as a cofactor for heparin. Heparin enhances the activity of antithrombin, allowing it to inhibit thrombin and other clotting factors more effectively, thereby regulating blood coagulation and preventing excessive clotting. This interaction significantly accelerates the anticoagulant effects of antithrombin, making it much more efficient at preventing thrombus formation in the bloodstream.

While antithrombin does play roles that might relate to other statements, those do not accurately capture its key functional characteristic. For instance, antithrombin is synthesized primarily in the liver, not megakaryocytes, which are responsible for producing platelets. Additionally, it is not activated by protein C; rather, protein C's activation cooperatively works to inactivate factors Va and VIIIa but does not initiate antithrombin's activity. Lastly, referring to antithrombin as a pathological inhibitor is misleading, as it functions normally to regulate coagulation rather than serve as a pathological entity in this context. Thus, the role of antithrombin as a cofactor for heparin is crucial in maintaining the