APLS (antiphospholipid antibody syndrome): An immune disorder characterized by the presence of abnormal antibodies in the blood associated with abnormal blood clotting, migraine headaches, recurrent pregnancy losses (repeat spontaneous abortions), and low blood platelet counts (thrombocytopenia). The abnormal antibodies are directed against phospholipids, fats that contains phosphorous.

APLS can occur by itself (primary APLS) or be caused by another underlying condition (secondary APLS). About a third of persons with primary APLS have heart valve abnormalities.

Antiphospholipid antibodies reduce the levels of annexin V, a protein that binds phospholipids and has potent anticoagulant activity. The reduction of annexin V levels is thought to be a possible mechanism underlying the increased tendency of blood to clot and the propensity to pregnancy loss characteristic of the antiphospholipid antibody syndrome.