Well, if you removed a kidney from a stallion and transplanted it into mare, she would reject it. First, she would identify it as being a foreign – something not from her own body – and then her immune cells would attack and destroy it. This is how immune systems work – detection of foreignness leading to various attack mechanisms.
Now think about what would happen if the mare instead mated with the stallion and conceived a foal which implanted inside her uterus. First of all, foal and mare would form a placenta – just about the most intimate biological interaction which ever occurs between two separate animals. Think about what the placenta is actually for.
A particular potential problem for the developing fetus is the proteins encoded by a small group of (up to six) genes called the ‘Major Histocompatibility Complex’. MHC proteins have particular properties which mean that they are they main reason why tissue grafts are rejected. Thus a fetus’s own MHC could potentially be detected and attacked by its mother’s immune system.
The exciting thing about pregnancy immunology is that we still don’t know exactly why the fetus is not rejected by its mother – this is very much a work in progress, although we do have some good ideas. It is clear that pregnancy is very much the ‘exception’ in the world of the immune system – a foreign ‘parasite’ being allowed to survive within a ‘host’ – and this may tell us all sorts of important things about how our body defends itself from disease.