The placenta has a unique structural organization that allows fetal cells expressing paternal alloantigens to establish a peaceful cohabitation with the maternal immune system. The fetal cells are continuously exposed to the humoral and cellular components of the maternal immune system present in the maternal blood that circulates in the intervillous space and in the decidual vessels. This review deals with the role played by the complement system at the placental level both in physiological and pathological conditions of pregnancies. Complement components found in the placental tissue derive to a large extent from blood circulating in placental vessels. However, some complement components may also be produced locally by macrophages and other cell types. Deposition of complement components at tissue level is usually found in association with inflammatory diseases. This is not the case in placentae in which deposits of complement components can also be documented in physiological conditions not resulting in fetal damage. Protection of the semiallogenic human conceptus against maternal complement activation products is achieved by surface expression of complement regulators that act at different steps of the complement sequence. These complement regulators are localized in a strategic position on the surface of villous trophoblast protecting the fetus from the damage that may derive from uncontrolled complement activation. However, pathological conditions of pregnancies may lead to deposition of a higher amount of complement activation products that may exceed the protection of local complement regulators.
