Antiphospholipid antibodies (aPL) have been shown to induce thrombosis, activate endothelial cells, and induce fetal loss. The pathogenesis of aPL‐induced thrombosis, although not completely understood, may involve platelet and endothelial cell activation as well as procoagulant effects of aPL directly on clotting pathway components. Recent studies have shown that uncontrolled complement activation leads to fetal death in aPL‐treated mice. In this study, we tested the hypothesis that aPL are responsible for activation of complement, thus generating split products that induce thrombosis.

To study thrombus dynamics and adhesion of leukocytes we used in vivo murine models of thrombosis and microcirculation, in which injections of aPL were used.

Mice deficient in complement components C3 and C5 were resistant to the enhanced thrombosis and endothelial cell activation that was induced by aPL. Furthermore, inhibition of C5 activation using anti‐C5 monoclonal antibodies prevented thrombophilia induced by aPL.

These data show that complement activation mediates 2 important effectors of aPL, induction of thrombosis and activation of endothelial cells.