Dimerization of β2GPI by anti-β2GPI autoantibodies from humans and mice that spontaneously develop APS potentiate thrombus formation in vivo which is attenuated by the A1-A1 inhibitor. A1-A1, inhibitor of dimeric β2GPI immune complexes; DI-DV, domain 1-5 of β2GPI. (Symbols) Green prong, anti-β₂GPI autoantibodies; red circle with minus, negative-charged phospholipid; gray half circle (U shape), ApoER2.