Human bodies readily reject organ transplant which is why immunosuppressants are prescribed to people who have to undergo transplants. However, a fetus is not rejected by the mother's immune system. How the body makes this decision has been studied for long. Now, a new discovery has answered this age-old question.

It turns out that as soon as the embryo implants itself, it sets of a mechanism that turns off the immune response against it. Thus the immune response does not recognize the embryo as a foreign body.

"Our manuscript addresses a fundamental question in the fields of transplantation immunology and reproductive biology, namely, how do the fetus and placenta, which express antigens that are disparate from the mother, avoid being rejected by the maternal immune system during pregnancy? What we found was completely unexpected at every level," said Dr. Adrian Erlebacher, associate professor of pathology and a member of the NYU Cancer Institute at NYU Langone Medical Center and lead author of the study.

Researchers say that chemokines are responsible for triggering the immune response. During pregnancy, these chemokine genes are shut down. If the chemokines were allowed to respond normally, they could accumulate the T-cells and kill the enemy-fetus.

Researchers had known for a fact that a barrier exists between maternal immune system and the fetus. The present study on animal models gave them the mechanism by which this barrier functions. The decidua encases the fetus and placenta and prevents the immune system attack by shutting down the chemokine genes.

"These findings give insight into mechanisms of fetal-maternal immune tolerance, as well as reveal the epigenetic modification of chemokine genes within tissue stromal cells as a modality for limiting the trafficking of activated T cells," Dr. Erlebacher said. "It turns out that the cells that typically secrete the chemoattractants to bring the T cells to sites of inflammation are inhibited from doing so in the context of the pregnant uterus. The decidua appears instead as a zone of relative immunological inactivity."

Previous research said that the fetus sends out some cells in the mother's blood. The maternal immune system destroys these migrating cells and not the fetus itself.

This research might help in understanding how a tumor grows inside the body and stay protected against the immune system.

"This is a very exciting finding for us because it gives a satisfying explanation for why the fetus isn't rejected during pregnancy, which is a fundamental question for the medical community with clear implications for human pregnancy. It also reveals a new modality for controlling T cell trafficking in peripheral tissues that could provide insight into a myriad of other conditions and diseases," said Dr. Erlebacher.

The study was published in journal Nature Immunology.