Mechanism describing the binding of antiphospholipid antibodies toβ₂-GPI. Our proposed mechanism is based on our findings in this study together with both the crystal structure and the NMR structure of β₂-GPI in solution: (1) Antiphos-pholipid antibodies cannot bind β₂-GPI in solution because epitope G40-R43 is covered by one of the carbohydrate chains. (2) Binding to a phospholipid membrane induces a conformational change in β₂-GPI. (3) As a result, the carbohydrate chain is not able to cover epitope G40-R43 anymore and is now able to bind antiphospholipid antibodies.