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Antiphospholipid syndrome (APS) is an autoantibody-mediated acquired thrombophilia characterized by recurrent arterial or venous thrombosis and/or pregnancy morbidity. The major autoantibodies detected in the patient’s sera are directed against phospholipid (PL)-binding plasma proteins, mainly against a 43-kDa plasma apolipoprotein known as β2 glycoprotein I (β2GPI) and prothrombin. The plasma concentration of β2GPI is 50–200 μg/mL. β2GPI consists of 326 amino acids arranged in five domains (I through V). Domain V forms a positively charged patch, suitable to interact with negatively charged PL. In plasma, β2GPI has a circular conformation with domain V binding to and concealing the B cell epitopes lying on domain I. Another group of antibodies termed lupus anticoagulant (LA) elongate clotting times in vitro; this elongation is not corrected by adding normal plasma to the detection system (Table 379-1). Patients with APS often possess antibodies recognizing Treponema pallidum PL/cholesterol complexes, which are detected as biologic false-positive serologic tests for syphilis (BFP-STS) and Venereal Disease Research Laboratory (VDRL) tests. APS may occur alone (primary) or in association with any other autoimmune disease (secondary). Catastrophic APS (CAPS) is defined as a rapidly progressive thromboembolic disease involving simultaneously three or more organs, organ systems, or tissues leading to corresponding functional defects.


Anti-PL (aPL)-binding plasma protein antibodies occur in 1–5% of the general population. Their prevalence increases with age; however, it is questionable whether they induce thrombotic events in elderly individuals. One-third of patients with systemic lupus erythematosus (SLE) (Chap. 378) possess these antibodies, whereas their prevalence in other autoimmune connective tissue disorders, such as systemic sclerosis (scleroderma), Sjögren’s syndrome, dermatomyositis, rheumatoid arthritis, and early undifferentiated connective tissue disease, ranges from 6 to 15%. One-third of aPL-positive individuals experience thrombotic events or pregnancy morbidity.

TABLE 379-1Classification and Nomenclature of Antiphospholipid Antibodies

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