The antiphospholipid syndrome is an autoimmune disorder of hypercoagulability characterized by the presence of autoantibodies to various phospholipids or phospholipid-binding proteins. The autoantibodies include anticardiolipin antibodies, lupus anticoagulant antibodies, and antibodies to β₂-glycoprotein I (a phospholipid-binding protein). These autoantibodies have both procoagulant and anticoagulant effects, but the procoagulant effects predominate, resulting in syndromes of venous and arterial thrombosis and pregnancy loss.