Annexin A5

Annexin A5 (or annexin V) is a cellular protein in the annexin group. The function of the protein is unknown, however annexin A5 has been proposed to play a role in the inhibition of blood coagulation by competing for phosphatidylserine binding sites with prothrombin and also to inhibit the activity of phospholipase A1 also by competing for phosphatidylserine binding sites. These properties have been found by in vitro experiments (in a test tube) and they have not been confirmed by experiments with laboratory animals.

Annexin A5 in pathology
Antibodies directed against annexin A5 are the cause of a syndrome called the antiphospholipid syndrome.

Annexin A5 forms a shield around negatively-charged phospholipid molecules. The formation of an annexin A5 shield blocks the entry of phospholipids into coagulation (clotting) reactions. In the antiphospholipid antibody syndrome, the formation of the shield is disrupted by antibodies. Without the shield, there is an increased quantity of phospholipid molecules on cell membranes, speeding up coagulation reactions and causing the blood-clotting characteristic of the antiphospholipid antibody syndrome.

Laboratory use of annexin A5
Annexin A5 is used as a probe in the annexin A5 affinity assay to detect cells that have expressed phosphatidylserine on the cell surface, a feature found in apoptosis as well as other forms of cell death.