Annexin V is an anticoagulant protein that acts as an indirect inhibitor of the thromboplastin-specific complex, which is involved in the blood coagulation cascade. Annexin V competes for phosphatidylserine binding sites (e.g. platelets) with prothrombin or coagulation factors and inhibis the activity of phospholipase A1. Antibodies directed against annexin V are found in patients with a disease called the antiphospholipid syndrome(APS), a thrombophilic disease associated with autoantibodies against phospholipid compounds. Annexin V is used as a probe in the "annexin V affinity assay" to detect cells that have expressed phosphatidylserine on the cell surface, a feature found in apoptosis and other forms of cell death.