Maternal immune tolerance

Immune tolerance in pregnancy or gestational/maternal immune tolerance is the absence of a maternal immune response against (in other words, immune tolerance towards) the fetus and placenta during pregnancy, which thus may be viewed as unusually successful allografts, since they genetically differ from the mother. In the same way, many cases of spontaneous abortion may be described in the same way as maternal transplant rejection. It is studied within the field of reproductive immunology.

The placenta functions as an immunological barrier between the mother and the fetus, creating an immunologically privileged site. For this purpose, it uses several mechanisms:

Still, the placenta does allow maternal IgG antibodies to pass to the fetus to protect it against infections. However, these antibodies do not target fetal cells, unless any fetal material has escaped across the placenta where it can come in contact with maternal B cells and make those B cells start to produce antibodies against fetal targets. The mother does produce antibodies against foreign ABO blood types, where the fetal blood cells are possible targets, but these preformed antibodies are usually of the IgM type, and therefore usually do not cross the placenta. Still, rarely, ABO incompatibility can give rise to IgG antibodies that cross the placenta, and are caused by sensitization of mothers (usually of blood type 0) to antigens in foods or bacteria.

Still, the placental barrier is not the sole means to evade the immune system, as foreign fetal cells also persist in the maternal circulation, on the other side of the placental barrier.

The placenta does not block maternal IgG antibodies, which thereby may pass through the human placenta, providing immune protection to the fetus against infectious diseases.

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