Normally, the role of the immune system is to fight off invading cells such as bacteria, viruses, or to reject alien tissues such as transplanted organs, that have a different genetic pattern to one’s own body. A foetus also has a different genetic pattern because it carries half the genes from the father. An embryo or early pregnancy is therefore at risk of rejection. In pregnancy, a woman’s immune system is naturally modulated in such a way that it accepts the father’s genes. This is a very complex process and can sometimes go wrong leading to difficulty in getting pregnant or leads to miscarriage.