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DEFINITIONS

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Antiphospholipid syndrome (APS) is an autoantibody-mediated acquired thrombophilia characterized by recurrent arterial or venous thrombosis and/or pregnancy morbidity. It affects primarily females. APS may occur alone (primary) or in association with other autoimmune diseases, mainly systemic lupus erythematosus (SLE) (secondary). Catastrophic APS (CAPS) is a life-threatening rapidly progressive thromboembolic disease involving simultaneously three or more organs.

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The major autoantibodies detected in patients’ sera are directed against phospholipids and/or phospholipid (PL)-binding plasma proteins such as prothrombin and β2 glycoprotein I (β2GPI). PLs are components of the cytoplasmic membrane of all living cells. The antibodies are directed against negatively charged PLs including among others cardiolipin, phosphocholine, and phosphatidylserine. The plasma protein β2GPI is a 43-kDa plasma apolipoprotein, which 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 PLs. 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) prolongs clotting times in vitro, which are not corrected by adding normal plasma (Table 350-1). Patients with APS often possess antibodies recognizing Treponema pallidum PL/cholesterol complexes, detected by Venereal Disease Research Laboratory (VDRL) tests and characterized as biologic false-positive serologic tests for syphilis (BFP-STS).

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TABLE 350-1Classification and Nomenclature of Antiphospholipid Antibodies
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EPIDEMIOLOGY

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The incidence of APS is estimated to be around 5 cases per 100,000 persons per year. Anti-PL antibodies occur in 1–5% of the general population. Their prevalence increases with age; however, it is questionable whether they are able to induce thrombotic events in elderly individuals. Moreover, one-third of patients with SLE and other autoimmune diseases (Chap. 349) possess these antibodies, ...

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