A condition in which the immune system mistakenly attacks normal proteins in the blood.

Antiphospholipid syndrome can cause blood clots to form within the arteries, veins and organs. The clots may lead to heart attack or stroke. The condition also can cause miscarriage and stillbirth in pregnant women.

Blood clotting related to this syndrome may cause pain, swelling, or redness in the leg or arm. A stroke can happen in a young person with antiphospholipid syndrome, even with no known risk factors for cardiovascular disease. It also may cause vaginal spotting or bleeding during pregnancy.

Blood-thinning medicines can reduce the risk of blood clots and miscarriage.