Autoimmune Protein S Deficiency in a Boy with Severe Thromboembolic Disease

Protein S is a vitamin K-dependent plasma protein involved in the regulation of the protein C anticoagulant pathway,1 the system responsible for the inactivation of factors Va and VIIIa2. Protein S functions as a cofactor for activated protein C by increasing its affinity for the cell surface3 and by blocking the capacity of factor Xa to protect factor Va from inactivation by activated protein C4; it also enhances the profibrinolytic effects of activated protein C5–7. The congenital deficiency of protein S is associated with an increased risk of recurrent juvenile venous and arterial thromboembolism8–10. . . .