What prevents a pregnant woman's immune system from recognizing her fetus as nonself (and attacking)?
Just so that someone answers this question so that the moderators have less work to do: 

The fundamental reason why embryos don't get massacred by the maternal immune system is because of the placenta and several of its functions. Three mechanisms are:

1) Secretion of neurokinin B, which is also secreted by parasites to avoid detection of the host. 
2) The placenta has developed immunoevasive actions from a virus. It creates a syncytium to limit transaction of mobile immune cells. 
3)It only allows specific antibodies,(IgG) to enter and bind infection. (IgG being the only isotype here). 

Yay  for biology