Immune system and pregnancy

Me, eight months pregnant

The immune system defends our body from alien organisms (or part of them). Sometimes it works properly, fighting bacteria, which can cause diseases. Other times it is too sensitive and recognize as “non-self”, and therefore dangerous, some harmless particles such as pollen, causing allergies.

When a woman is pregnant, an “alien” organism grows inside her.

A fetus is, indeed, alien to the mother’s body. So, how is it possible that the mother’s immune system does not attack the fetus?

The answer to this question was given last year by Vento-Tormo and colleagues. They published a very interesting paper on Nature in which they reveal the complexity in maternal-fetal communication during the early stages of pregnancy.

In the first days of pregnancy (when women don’t even know about their new condition), the newly formed embryo moves into the decidua. The decidua forms the maternal part of the placenta. To finalize the formation of the placenta and all connections between mother and fetus, it is necessary that some embryonic cells invade mother’s tissue.

The researchers studied 70000 cells from placental and decidual tissues of women, who had an abortion between 6 and 14 weeks. They assessed the gene activity of each cell and identified 35 different types of cell.

Among them, they identified some natural killer cells from the maternal side. These kind of cells usually help fight cancer, but during pregnancy they work as peacekeepers. They prevent other mother’s immune system cells from attacking the fetus. Moreover, they produce some compounds that help fetal growth and the establishment of blood vessels.

The researchers identified several mechanisms that contribute to the formation of a physiologically peaceful environment.

These discoveries could help to better understand the communication between mother’s and fetus’ immune system and, therefore, help reduce miscarriages and other complications.


Vento-Tormo et al., “Single-cell reconstruction of the early maternal–fetal interface in humans”, Nature, (2018), 563:347-353,

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