Why patients develop anti-phospholipid autoantibodies (aPL) is not completely understood. The production of these autoantibodies likely is triggered by an environmental factor, such as an infection occurring in an individual with a genetic background that makes him or her more susceptible to the disease. The aPL can be present in the bloodstream for a long time, but thrombotic events result only occasionally. aPL increases the risk for blood clotting, but thrombosis usually occurs when other conditions that favor clotting are present, such as prolonged inactivity (e.g., being restricted to bed), surgery, or pregnancy. Additional risk factors for thrombosis are hypertension, obesity, smoking, atherosclerosis (hardening of the arteries), the use of estrogens (birth control pills), and an associated systemic autoimmune disease (mainly SLE or SLE-like diseases)
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