Annexin V, also known as placental anticoagulant protein (PAP I), not only belongs to the family of calcium-dependent proteins that bind phospholipids, but is also a possible vascular anticoagulant protein. The cause of miscarriage in the middle and late stages with the progression of antiphospholipid syndrome (AFS) is the development of thrombotic vasculopathy of the spiral arteries of the placenta. The role of annexin V in the. the genesis of fetal loss syndrome in patients with APS, whose reduction on the surface of villi leads to the development of thrombosis and infarction of the placenta. Annexin V is present in many tissues, mainly on endothelial cells and the placenta. At low concentrations annexin V is present in platelets, in higher concentrations in erythrocytes and leukocytes. It has pronounced anticoagulant properties, which is due to its high affinity for anionic phospholipids and the ability to prevent activated blood clotting factors from binding to phospholipids of cell membranes. The affinity of annexin V to phosphatidylserine (a type of negatively charged phospholipid) is 1000 times higher than that of prothrombin. The plasma membrane of a cell is characterized by an asymmetric structure, and its disappearance is the first sign of apoptosis. Under normal conditions, phosphatidylserine is localized on the cytoplasmic side of the cell membrane, while in apoptosis it is located on the outer surface of the membrane, where it binds highly specifically to annexin V. Annexin V covers phosphatidylserine as a carpet, with a local anticoagulant effect. Therefore, during physiological pregnancy, despite the long-term location of phosphatidylserine on the surface of the trophoblast, there is no permanent thrombus formation. In patients with APS, including against the background of an infectious process, the production of autoantibodies to this protein is possible. These antibodies displace annexin V from the surface of endotheliocytes and trophoblast cells, leading to hypercoagulation and pregnancy loss. As pregnancy progresses, the processes of thrombus formation in the vessels of the placenta become more important. The presence of antibodies to annexin V can be associated with arterial and venous thrombosis, the development of rheumatoid arthritis, systemic lupus erythematosus and persistent miscarriage. Antibodies to annexin V are not included in the laboratory criteria of APS, but are one of the auxiliary ("non-criteria") APS markers and are found in 15-20% of cases.