Why don't mothers reject their foetus?

A foetus is created from a mixture of its mother's and father's genes so the baby is genetically unique. So, what is it that protects babies from their mum's immune system. Ziyad Yehia asked Dr. Lucy Fairclough, an immunologist at the University of Nottingham, just how this works.

Lucy - The immune system normally functions to remove anything foreign from the body such as an infection. In the case of pregnancy, the mother's immune system will see the baby as foreign because the baby is genetically made up from the mother and the father. However, in most cases, the mother does not reject the baby.

Ziyad - What allows mums to put up with this alien life form inside of her for so long? Well, besides a lot of patience and random food.

Lucy - Well, at the side of the placenta, there is contact between the foetal and maternal tissues. But at the placenta, there is maternal tolerance of the foetal tissue despite these genetic differences.

Ziyad - I think a lot of expectant mothers would definitely say they tolerate a lot during pregnancy. Just how do they do it, Lucy?

Lucy - There are many mechanisms that help to maintain this tolerance. This can include the secretion of special proteins that suppress the mother's immune system or an accumulation of cells that regulate and keep the immune system in check. There can also be lack of other proteins that normally activate the immune system. All of these causes a local suppression of immunity during pregnancy that enables the foetus to survive even though it was genetically different than mother.

Ziyad - Sounds like mums got it covered, but nobody is perfect, right?

Lucy - There are some instances when the foetus may be attacked by the mother's immune system although this is not controlled by careful screening. There are many proteins expressed on the blood cells of the body and an individual's blood type is defined by some of these proteins. In the case of pregnancy, when the father's red blood cells express a protein called rhesus D, but the mothers does not express this rhesus D protein, the mother generates a molecule called an antibody that targets the foetus's red blood cells. This is called haemolytic disease of the new born. In most cases however, the red blood cells do express rhesus D, so this is a fairly rare occurrence.