The way the human immune system works is pretty well known. It only goes into activation when a foreign invader is present, then it rushes antibodies and other chemicals in to destroy a virus, bacteria, fungus, or parasite. A virus invades our cells, so the immune system destroys those infected cells. That can be pretty destructive, which is why our immune system stays dormant until we need it.
However, in a pregnant mother's placenta, the immune system works differently. It switches on even when there is no infectious agent present. To do this, it manufactures its own "fake virus" from its DNA! This causes the placenta to produce interferon lambda and activates a low-level, gentle immune response that does not attack cells the way a normal immune response would. This system keeps the fetus' responses always on, ready to fight any possible infection that the mother may contract. That's important, since the mothers' immune response is weakened during pregnancy to keep her body from rejecting the fetus as an invader.
Read up on this new discovery and how it was found at Quanta magazine. -via Damn Interesting
(Image credit: Kristina Armitage/Quanta Magazine)