We have shown that phagocytosis of trophoblast debris from normal placentae alters the phenotype of macrophages such that they are likely to deviate maternal immune responses towards tolerance and away from inflammation. This may be one of the mechanisms that allow the human fetal allograft to survive in direct contact with the maternal immune system...
So in normal pregnancies, when your white blood cells sweep up the bits of material that are released into your bloodstream by the placenta, that turns down inflammatory cytokines and turns up anti-inflammatory cytokines. The placenta is signalling the maternal immune system to calm the heck down.
In our pregnancies, this does not happen properly.