Experimental approach. APS sera was used as a source of APS IgG, which was purified using protein A/G beads. APS IgG was then either tested directly for its effects on thrombosis after laser-induced vascular injury (in which case a minimum of 400 μg was required to induce thrombosis), or used as a source for affinity purification of anti-β₂GPI antibodies using immobilized β₂GPI. The ability of anti-β₂GPI antibodies to enhance thrombosis was then assessed after infusion of anti-β₂GPI antibodies into mice (only 1.5-3.0 μg of IgG was required to enhance thrombosis). Professional illustration by Kenneth X. Probst.