Antiphospholipid syndrome is a disorder characterized by an increased tendency to form abnormal blood clots (thromboses) that can block blood vessels. This clotting tendency is known as thrombophilia. In antiphospholipid syndrome, the thromboses can develop in nearly any blood vessel in the body. If a blood clot forms in the vessels in the brain, blood flow is impaired and can lead to stroke. Antiphospholipid syndrome is an autoimmune disorder. Autoimmune disorders occur when the immune system attacks the body's own tissues and organs.

The genetic cause of antiphospholipid syndrome is unknown. This condition results from the presence of three abnormal immune proteins (antibodies) in the blood. The antibodies that cause antiphospholipid syndrome are called lupus anticoagulant, anticardiolipin, and anti-B2 glycoprotein I. These antibodies are referred to as antiphospholipid antibodies. People with this condition can test positive for one, two, or all three antiphospholipid antibodies in their blood. Antibodies normally attach (bind) to specific foreign particles and germs, marking them for destruction, but the antibodies in antiphospholipid syndrome attack normal human proteins. When these antibodies attach to proteins, the proteins change shape and attach to other molecules and receptors on the surface of cells. Attaching to cells, particularly immune cells, turns on (activates) the blood clotting pathway and other immune responses.

Antiphospholipid syndrome is estimated to affect 1 in 2,000 people. This condition may be responsible for up to one percent of all thromboses. It is estimated that 20 percent of individuals younger than age 50 who have a stroke have antiphospholipid syndrome. Ten to 15 percent of people with systemic lupus erythematosus have antiphospholipid syndrome. Similarly, 10 to 15 percent of women with recurrent miscarriages likely have this condition. Approximately 70 percent of individuals diagnosed with antiphospholipid syndrome are female.

Most cases of antiphospholipid syndrome are sporadic, which means they occur in people with no history of the disorder in their family. Rarely, the condition has been reported to run in families; however, it does not have a clear pattern of inheritance. Multiple genetic and environmental factors likely play a part in determining the risk of developing antiphospholipid syndrome.

Published Date: July 11, 2022Published By: National Institutes of Health