Antiphospholipid syndrome (APS) is an autoimmune disease with various clinical manifestations, including arterial and venous thrombosis, thrombocytopenia, recurrent fetal loss as well as cardiac, hematological and neurological manifestations. In its worst manifestation, APS is associated with disseminated thrombus formation in a variety of vessels leading to multi-organ failure and thrombocytopenia.

The presence of antiphospholipid antibodies is a central serological finding in APS, consequently they play a critical role in its diagnosis, i.e. they target several phospholipid binding plasma proteins, including beta-2-glycoprotein I (b2-GPI) and prothrombin.

Cardiolipin is the most important but not sole phospholipid antigen, as antibodies to other phospholipids such as phosphatidylserine, -choline, -ethanolamine are also present in APS patients, emphasizing that antiphospholipid antibodies represent an extremely heterogenous group of autoantibodies. Optimal detection of antiphospholipid antibodies in ELISA tests is achieved through combining cardiolipin with the cofactor ß2-glycoprotein I (apolipoprotein H).  Only the binding of the phospholipid to ß2-glycoprotein I results in the formation of the actual antigen which is targeted by the cardiolipin antibodies.