What is antiphospholipid syndrome (APS)?

Antiphospholipid syndrome (APS), also known as antiphospholipid antibody syndrome and sometimes Hughes syndrome, is a disorder characterized by elevated levels of multiple different antibodies (proteins produced by the body to fight off foreign substances) that are associated with both arterial and venous thrombosis (clots in the arteries and veins).

There are two primary classes of antiphospholipid (aPL) antibodies, the antibodies associated with APS. These are called anticardiolipin antibodies and the lupus anticoagulant, and are directed against specific molecules. These aPL antibodies appear to be mainly directed against two particular molecules: beta-2-glycoprotein I (ß2GPI, a normal protein found in the blood whose function is unknown) and another molecule known as prothrombin (a normal blood protein that binds to phospholipids and plays a very important role in blood clotting).

These aPL antibodies were first noted in a group of people who had positive tests for syphilis without signs of infection. It was then noticed that some individuals who continued to have false-positive tests for syphilis went on to develop systemic lupus erythematosus (SLE) and other similar conditions. Later studies found a protein called the lupus anticoagulant in a number of individuals with SLE. A case report in 1956 described an individual with recurrent pregnancy loss, thrombophlebitis (inflammation of a vein related to a blood clot) and lupus anticoagulant. The work of Dr. Graham Hughes and his colleagues in the 1980s provided further understanding of APS, including the introduction of testing for anticardiolipin antibodies.