Immune System Of Mother

Further information: Immune tolerance in pregnancy
The placenta and fetus may be regarded as a foreign allograft inside the mother, and thus must evade from attack by the mother's immune system.
For this purpose, the placenta uses several mechanisms:
It secretes Neurokinin B-containing phosphocholine molecules. This is the same mechanism used by parasitic nematodes to avoid detection by the immune system of their host.
There is presence of small lymphocytic suppressor cells in the fetus that inhibit maternal cytotoxic T cells by inhibiting the response to interleukin 2.
However, the Placental barrier is not the sole means to evade the immune system, as foreign foetal cells also persist in the maternal circulation, on the other side of the placental barrier.