Science Questions

Why don't mothers reject their foetus?

Mon, 8th Jun 2015

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Lorianna asked:

I was wondering why the mother doesn't reject the baby because it has a different genetic sequence?


A foetus is created from a mixture of its motherís and fatherís genes so the baby isBaby in Womb genetically unique. So, what is it that protects babies from their mumís 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 motherís 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 motherís 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 motherís immune system although this is not controlled by careful screening. There are many proteins expressed on the blood cells of the body and an individualís blood type is defined by some of these proteins. In the case of pregnancy, when the fatherís 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 foetusís 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.



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Often the fetus IS rejected.

"Around half of all fertilized eggs die and are lost (aborted) spontaneously, usually before the woman knows she is pregnant. Among women who know they are pregnant, the miscarriage rate is about 15-20%. Most miscarriages occur during the first 7 weeks of pregnancy. The rate of miscarriage drops after the baby's heart beat is detected."

US National Library of Medicine

That's why Anne Boleyn got topped, you know. Pecos_Bill, Mon, 8th Jun 2015

This question was referring more specifically to the immune rejection of a developing foetus and what prevents this from happening, given the close contact between foetal and maternal tissues. Your point about early failure of embryos to implant is a good one though and points to a genetic selection process. chris, Sun, 14th Jun 2015

But can we be sure that the immune system is innocent in <<all>> if not most aspects of fetal wastage? If there is a genetic problem, what would suss that out but the immune system?

Certainly the mother's immune system is modified by normal cyesis.  "Pregnant women are considered to be a special population group due to their specific susceptibility to some infectious diseases because of the unique Ďimmunologicalí condition caused by pregnancy. " (1)

If those mods don't mesh with the pregnancy properly -- it might not be auspicious for either part of the dyad.

In my callow youth perinatal field nursing among "undocumented" hispanic women (with increased morbidity} was a large part of my rice bowl. In that population immunological screening? I once spent a whole day searching out an MD for a post dates prima para with NO prenatal care --- who was <<white>>. If I were not retired I would expect to find the same mileu in California today.

1. The Immune System in Pregnancy: A Unique Complexity,
Pecos_Bill, Sun, 14th Jun 2015

I've read that humans and other placental mammals make use of an endogenous virus carried in our DNA that is activated within the placenta. I had the impression that the virus had mastered the art of hiding from the immune system and our ancestors incorporated it as a tool enabling foetal growth within the body without sealing the baby in an egg.

Is this not correct? AndroidNeox, Tue, 27th Oct 2015

I would guess that the ovum, by containing the lion's share of the material needed to form the fetus, by being formed by the mother is is accepted by her body. The male DNA, upon fertilization, is only a small fraction of the total mass and becomes hidden away from her immune system, which sees only the bulk mass.

This hidden effect is connected to the mother originally having female DNA from her mother and male DNA from her father. When the ovum matures, the ovum extrudes half the DNA. When the male DNA from the sperm enters the ovum, the differences with the original full DNA is subtle; disguised and hidden.
puppypower, Thu, 29th Oct 2015

Another question in a similar vein (excuse the pun): why doesn't the immune system reject blood transfusions? vhfpmr, Mon, 16th Nov 2015

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