Evolution of mammalian pregnancy in the presence of the maternal immune system.

Eutherian mammals have inherited a typical vertebrate immune system, which protects the body against infectious organisms by detecting and destroying foreign biological material. However, with the evolution of longer gestation periods, this protective mechanism became a potential threat to the 'semi-foreign' fetus and so eutherians have developed systems to prevent immune rejection of their developing fetuses. In many species, this is achieved by reducing placental expression of major histocompatibility complex (MHC) genes, the products of which are responsible for most transplantation rejection reactions. Unexpectedly, however, major histocompatibility complex expression is often re-established in the most invasive trophoblast cells. It is not known why transplantation antigen expression in the fetal cells most exposed to the maternal immune system is advantageous. It is possible that such expression aids the process of invasion or exerts an immunoprotective effect on the fetus. It may prove possible to identify the essential steps that all eutherian fetuses take to ensure their survival in the face of potential maternal immune attack by studying the common features of the placental immunology of different species.