What Causes Antiphospholipid Antibody Syndrome?

Antiphospholipid antibody syndrome (APS) occurs when 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 and 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 occurs. Some believe that the antibodies damage or affect the inner lining of the blood vessels, causing blood clots to form. Others believe that the immune system makes antibodies in response to blood clots damaging the blood vessels.