Why Isn’t A Developing Fetus Rejected Like A Tissue Graft?
If you are doing Biology at school, you will probably have learnt about the immune system, and a little bit about pregnancy, too. However, one thing which may never have crossed your mind is that pregnancy causes a potential conflict with the immune system. Why?
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 (see Activity 1).
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 (see Activity 2). 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 (see Activity 3) 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 (see activity 4). 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.
Once you have looked at these resources, think about and research the following questions. Be aware that we don’t really know the answers to some of these things ourselves!
1. Why does the placenta need an active immune system, anyway? Why can’t the immune system in the placenta just be ‘switched off’?
2. If avoiding the maternal immune system is so important, why do the fetuses of different mammalian species seem to have done it in such different ways?
3. If so many mares make antibodies to their foals, with no apparent ill-effects, why do fetuses try so hard to avoid maternal immune detection?
4. If the placenta does essentially the same thing in different species (does it?), why does it vary so much in its structure?
5. Some trivia! Can you find out where the word ‘fetus’ came from? Who was the first known person to misspell it ‘foetus’? Why do you think British people (especially non-scientists) still often misspell it?