Annexin V: Annexin V is the cause of a syndrome called the antiphospholipid antibody syndrome with abnormal blood clotting.

The annexins are a family of proteins first described in 1990. All of the annexin proteins share the property of binding calcium and phospholipids.

Annexin V normally forms a shield around certain phospholipid molecules that blocks their entry into coagulation (clotting) reactions. In the antiphospholipid antibody syndrome, the formation of this shield is disrupted by the abnormal antibodies. Without the shield, there is an increased quantity of phospholipid molecules on cell membranes, speeding up coagulation reactions and causing the abnormal blood clotting characteristic of the antiphospholipid antibody syndrome.