Why doesn't an expectant mother's immune system damage her growing baby - since it has 'foreign' DNA?

This fundamental phenomenon has yet to be fully explained.

During pregnancy, the maternal immune system is exposed to a major challenge. The fetus expresses paternal alloantigens, yet it is not rejected.
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"Pregnancy represents a major challenge to immunologic tolerance. How the fetal 'semiallograft' evades maternal immune attack is unknown."

Source : Journal of the American Society of Hematology vol. 117 no. 6 pp 1861-1868

It's known that a specialised type of immune response cell called uterine Natural Killer cells (uNK cells) play a part in suppressing the mother's immune response. They make up about 70% of the leukocytes (white blood cells) in the uterus during early pregnancy. Their precise function, method of action, and their origin, is currently unknown. (see International Journal of Developmental Biology54: 281 - 294)

Similarly, the developing baby also has its own (developing) immune system - which somehow accepts the presence of the mother's cells, even though they have differing DNA to its own.

Also see : Immunological toleranceplugin-autotooltip__plain plugin-autotooltip_bigImmunological tolerance

Immunological tolerance is the ability of an individual's immune system to ignore "self" while reacting to "non-self" organisms and substances.

The system is extremely complex, and needs to somehow set a balance between being able to accurately sense and eradicate "non-self" entities, whilst leaving the "self" cells etc. untouched.

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