Antiphospholipid (AN-te-fos-fo-LIP-id) syndrome is a condition in which the immune system mistakenly creates antibodies that attack tissues in the body. These antibodies can cause blood clots to form in arteries and veins.

Blood clots can form in the legs, lungs and other organs, such as the kidneys and spleen. The clots can lead to a heart attack, strokes and other conditions. During pregnancy, antiphospholipid syndrome also can result in miscarriage and stillbirth. Some people who have the syndrome have no signs or symptoms.

There's no cure for this uncommon condition, but medications can reduce the risk of blood clots and miscarriage.


Signs and symptoms of antiphospholipid syndrome can include:

  • Blood clots in legs (DVT). Signs of a deep vein thrombosis (DVT) include pain, swelling and redness. These clots can travel to the lungs (pulmonary embolism).
  • Repeated miscarriages or stillbirths. Other complications of pregnancy include dangerously high blood pressure (preeclampsia) and premature delivery.
  • Stroke. A stroke can occur in a young person who has antiphospholipid syndrome but no known risk factors for cardiovascular diseases.
  • Transient ischemic attack (TIA). Similar to a stroke, a transient ischemic attack (TIA) usually lasts only a few minutes and causes no permanent damage.
  • Rash. Some people develop a red rash with a lacy, net-like pattern.

Less common signs and symptoms include:

  • Neurological symptoms. Chronic headaches, including migraines; dementia and seizures are possible when a blood clot blocks blood flow to parts of the brain.
  • Cardiovascular disease. Antiphospholipid syndrome can damage heart valves.
  • Low blood platelet counts (thrombocytopenia). This decrease in blood cells needed for clotting can cause episodes of bleeding, particularly from the nose and gums. Bleeding into the skin will appear as patches of small red spots.

When to see a doctor

Contact your health care provider if you have unexplained bleeding from your nose or gums; an unusually heavy menstrual period; vomit that is bright red or looks like coffee grounds; black, tarry stool or bright red stool; or unexplained abdominal pain.

Seek emergency care if you have signs and symptoms of:

  • Stroke. A clot in your brain can cause sudden numbness, weakness or paralysis of your face, arm or leg. You may have difficulty speaking or understanding speech, visual disturbances and a severe headache.
  • Pulmonary embolism. If a clot lodges in your lung, you may experience sudden shortness of breath, chest pain and coughing up blood-streaked mucus.
  • Deep vein thrombosis (DVT). Signs and symptoms of DVTs include swelling, redness, or pain in a leg or arm.


Antiphospholipid syndrome occurs when the immune system mistakenly produces antibodies that make blood much more likely to clot. Antibodies usually protect the body against invaders, such as viruses and bacteria.

Antiphospholipid syndrome can be caused by an underlying condition, such as an autoimmune disorder. You can also develop the syndrome without an underlying cause.

Risk factors

Antiphospholipid syndrome is more common in women than in men. Having another autoimmune condition, such as lupus, increases the risk of antiphospholipid syndrome.

It's possible to have the antibodies associated with antiphospholipid syndrome without developing signs or symptoms. However, having these antibodies increases your risk of developing blood clots, particularly if you:

  • Become pregnant
  • Are immobile for a time, such as being on bed rest or sitting during a long flight
  • Have surgery
  • Smoke cigarettes
  • Take oral contraceptives or estrogen therapy for menopause
  • Have high cholesterol and triglycerides levels


Complications of antiphospholipid syndrome can include:

  • Kidney failure. This can result from decreased blood flow to your kidneys.
  • Stroke. Decreased blood flow to a part of your brain can cause a stroke, which can result in permanent neurological damage, such as partial paralysis and loss of speech.
  • Cardiovascular problems. A blood clot in your leg can damage the valves in the veins, which keep blood flowing to your heart. This can result in chronic swelling and discoloration in your lower legs. Another possible complication is heart damage.
  • Lung problems. These can include high blood pressure in your lungs and pulmonary embolism.
  • Pregnancy complications. These can include miscarriages, stillbirths, premature delivery, slow fetal growth and dangerously high blood pressure during pregnancy (preeclampsia).

Rarely, in severe cases, antiphospholipid syndrome can lead to multiple organ damage in a short time.

Feb 25, 2022

  1. Antiphospholipid antibody syndrome. National Heart, Lung, and Blood Institute. https://www.nhlbi.nih.gov/health-topics/antiphospholipid-antibody-syndrome. Accessed Nov. 9, 2021.
  2. Erkan D, et al. Clinical manifestations of antiphospholipid syndrome. https://www.uptodate.com/contents/search. Accessed Nov. 9, 2021.
  3. Antiphospholipid syndrome. American College of Rheumatology. https://www.rheumatology.org/I-Am-A/Patient-Caregiver/Diseases-Conditions/Antiphospholipid-Syndrome. Accessed Nov. 9, 2021.
  4. AskMayoExpert. Antiphospholipid antibodies and antiphospholipid syndrome. Mayo Clinic; 2021.
  5. Limper M, et al. Antiphospholipid syndrome: State of the art on clinical practice guidelines. Rheumatic and Musculoskeletal Diseases Open. 2018; doi:10.1136/rmdopen-2018-000785.
  6. Sayar Z, et al. Thrombotic antiphospholipid syndrome: A practical guide to diagnosis and management. Thrombosis Research. 2021; doi: org/10.1016/j.thromres.2020.10.010.
  7. Erkan D, et al. Treatment of the antiphospholipid syndrome. https://www.uptodate.com/content/search. Accessed Nov. 9, 2021.
  8. Blood thinner pills: Your guide to using them safely. Agency for Healthcare Research and Quality. http://www.ahrq.gov/patients-consumers/diagnosis-treatment/treatments/btpills/btpills.html. Accessed Nov. 9, 2021.


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