Definition: Word: Antithrombin, a protein. Definition: - Antithrombin is a protein that helps prevent the formation of a blood clot in the body by neutralizing the enzyme thrombin, which causes platelets to break down and release blood clots. - It plays a role in treating various types of bleeding disorders, such as hemophilia or von Willebrand disease. Example: In an emergency situation where the patient is at risk for bleeding into organs, antithrombin helps prevent the formation of a blood clot by preventing platelets from breaking down and allowing the body's natural response to clotting to occur.