b2GPI, also called apolipoprotein H, is an antiphospholipid protein cofactor with natural anticoagulant properties and an affinity for negatively-charged phospholipids. Antibodies directed against b2GPI have been shown to be specific markers for thrombosis in individuals with systemic lupus erythematosus (SLE) and lupus-like disorders (APS). Most autoimmune anti-phospholipid antibodies require the serum cofactor b2GPI for optimal binding. It has been shown that many anti-phospholipid antibodies may react to a neoepitope formed on the b2GPI molecule by the interaction between the phospholipid and b2GPI. The physiological role of b2GPI may additionally be to participate in apoptosis.