Adaptive Immunity During Pregnancy
The cornerstone of the immune system is the recognition of "self" versus "non-self". Therefore, the mechanisms which protect the human fetus (which is considered "non-self") from attack by the immune system, are particularly interesting. Although no comprehensive explanation has emerged to explain this mysterious, and often repeated, lack of rejection, two classical reasons may explain how the fetus is tolerated. The first is that the fetus occupies a portion of the body protected by a non-immunological barrier, the uterus, which the immune system does not routinely patrol. The second is that the fetus itself may promote local immunosuppression in the mother, perhaps by a process of active nutrient depletion. A more modern explanation for this induction of tolerance is that specific glycoproteins expressed in the uterus during pregnancy suppress the uterine immune response (see eu-FEDS).
During pregnancy in viviparous mammals (all mammals except Monotremes), endogenous retroviruses (ERVs) are activated and produced in high quantities during the implantation of the embryo. They are currently known to possess immunosuppressive properties, suggesting a role in protecting the embryo from its mother's immune system. Also viral fusion proteins apparently cause the formation of the placental syncytium in order to limit the exchange of migratory cells between the developing embryo and the body of the mother (something an epithelium will not do sufficiently, as certain blood cells are specialized to be able to insert themselves between adjacent epithelial cells). The immunodepressive action was the initial normal behavior of the virus, similar to HIV, the fusion proteins were a way to spread the infection to other cells by simply merging them with the infected one (HIV does this too). It is believed that the ancestors of modern viviparous mammals evolved after an infection by this virus, enabling the fetus to survive the immune system of the mother.
The human genome project found several thousand ERVs classified into 24 families.
Read more about this topic: Adaptive Immune System
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