β₂-GPI mediates complement-regulatory activities. Circular, inactive β₂-GPI is recruited from plasma to modified surfaces that expose oxidized phospholipids, such as cardiolipin. Circular β₂-GPI opens up and can bind C3, inducing a conformational change to enable binding of factor H and degradation by factor I. Elongated β₂-GPI also acts on C3b to enhance binding of factor H and degradation and inactivation by factor I.