An anomaly of the protein C anticoagulant pathway, which serves as a major system for controlling thrombosis, limiting inflammatory responses, and potentially decreasing endothelial cell apoptosis in response to inflammatory cytokines and ischemia. A natural anticoagulant system denoted the protein C pathway exerts its anticoagulant effect by regulating the activity of FVIIIa and FVa. The vitamin K-dependent protein C is the key component of the pathway. Activated protein C (APC) cleaves and inhibits coagulation cofactors FVIIIa and FVa, which result in downregulation of the activity of the coagulation system. The endothelial protein C receptor stimulates the T-TM-mediated activation of protein C on the endothelial cell surface. The two cofactors, protein S and the intact form of FV, enhance the anticoagulant activity of APC. [ http://www.ncbi.nlm.nih.gov/pubmed/15943976 http://www.ncbi.nlm.nih.gov/pubmed/12970121 ]