Mechanism of aPL-induced TF increase and fetal death. APL antibodies are preferentially targeted to the placenta where they activate complement leading to the generation of potent anaphylatoxin C5a. C5a attracts and activates neutrophils. As a result of C5a-C5aR interaction, neutrophils express TF. TF on neutrophils contribute to oxidative burst and subsequent trophoblast injury and ultimately fetal death.