Annexin A5 (or annexin V) is a cellular protein of the annexin family. 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. Annexin A5 forms a shield around negatively-charged phospholipid molecules, which blocks the entry of phospholipids into coagulation (clotting) reactions. Antibodies directed anti annexin A5 are the cause of the antiphospholipid syndrom in which 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. 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 aswell as other forms of cell death (1,2). Platelets also expose phosphatidyl-serine on their surface when activated, which serves as binding site for various coagulation factors. Annexin A5 has been shown to interact with kinase insert domain receptor (3) and integrin beta 5 (4). RUU-WAC2a is a monoclonal that specifically reacts with annexin A5. This monoclonal antibody is directed to the phospholipid binding domain of annexin A5 and as such it inhibits annexin A5 binding to anionic phospholipid membranes for more than 95%. RUU-WAC 2A does not remove annexin A5 bound to phospholipid surfaces indiCating that the epitope is not accessible after interaction with annexin A5.