Schematic representation of circular and open β₂GPI. (A) β₂GPI with its 5 domains (DI-DV) as it is in complex with anti-β₂GPI antibodies. (B) A proposed model of plasma β₂GPI. Black dots represent the amino acids not accessible for trypsin; black arrow indicates the location of the amino acids involved in the recognition of anti-β₂GPI antibodies. In the circular conformation, the black circle indicates the concealing of the amino acids involved in the binding site of anti-β₂GPI antibodies as a result of interaction between domains I and V of β₂GPI.