Antiphospholipid antibodies were originally thought to bind negatively-charged (aniomic) phospholipids. Current evidence suggests that the target antigen is considerably more complex and includes β₂-glycoprotein I, a phospholipid-binding plasma protein. Our under standing of the pathophysiology of the antiphospholipid syndrome has increased exponen tially with a number of studies into the interactions of antiphospholipid antibodies and β₂-glycoprotein I.