Antiphospholipid antibody syndrome is an autoimmune disorder in which patients have autoantibodies to phospholipid-bound proteins. Venous or arterial thrombi may occur. The pathophysiology is not precisely known. Diagnosis is by blood tests. Anticoagulation is often used for prevention and treatment.
(See also Overview of Thrombotic Disorders.)
The antiphospholipid antibody syndrome (APS) is an autoimmune disorder that consists of thrombosis and (in pregnancy) fetal demise caused by various antibodies directed against one or more phospholipid-bound proteins (eg, beta- 2glycoprotein 1, prothrombin, annexin A5).
Annexin A5 may bind to phospholipid membrane constituents to prevent the cell membrane from initiating the activation of coagulation. If autoantibodies displace annexin A5, procoagulant endothelial cell surfaces may be exposed and provoke arterial or venous thromboses.
The precise mechanism of thrombosis in patients with autoantibodies to phospholipid-bound beta2 glycoprotein 1 is unknown.
Results of in vitro clotting tests may paradoxically be prolonged because the autoantibodies to phospholipid-bound proteins interfere with coagulation factor assembly and activation on the phospholipid components added to plasma to initiate the tests. The lupus anticoagulant is an autoantibody that binds to phospholipid-bound protein complexes. It was initially recognized in patients with systemic lupus erythematosus (SLE), but these patients now account for only a minority of people with the autoantibody.
Other symptoms of venous or arterial thrombosis may also develop. Patients with autoantibodies to phospholipid-bound prothrombin may have levels of circulating prothrombin that are low enough to increase risk of bleeding. Some patients have thrombocytopenia.

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