Antiphospholipid antibody syndrome (APS) occurs if the body's immune system makes antibodies (proteins) that attack phospholipids.

Phospholipids are a type of fat found in all living cells and cell membranes, including blood cells and the lining of blood vessels. What causes the immune system to make antibodies against phospholipids isn't known.

APS causes unwanted blood clots to form in the body's arteries and veins. Usually, blood clotting is a normal bodily process. It helps seal small cuts or breaks on blood vessel walls. This prevents you from losing too much blood. In APS, however, too much blood clotting can block blood flow and damage the body's organs.

Researchers don't know why APS antibodies cause blood clots to form. Some believe that the antibodies damage or affect the inner lining of the blood vessels, which causes blood clots to form. Others believe that the immune system makes antibodies in response to blood clots damaging the blood vessels.