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What Causes Antiphospholipid Antibody Syndrome?

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. Researchers don’t know what causes the immune system to make antibodies against phospholipids.

APS causes 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. Clotting prevents you from losing too much blood. In APS, however, too much 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 clotting. Others believe that the immune system makes antibodies in response to blood clots damaging the blood vessels.

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Antiphospholipid Antibody Syndrome Clinical Trials

Clinical trials are research studies that explore whether a medical strategy, treatment, or device is safe and effective for humans. To find clinical trials that are currently underway for Antiphospholipid Antibody Syndrome, visit

May 17, 2012 Last Updated Icon

The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.