Researchers have discovered the fetal immune system is operational by the second trimester, with the discovery offering possible new insight into miscarriage.

International scientists have discovered the fetal immune system is active by the second trimester of pregnancy.

The new understanding may help reveal some causes of miscarriage, say immunologists.

Principal investigator Florent Ginhoux and colleagues at the Singapore Immunology Network studied tissues from 96 fetuses from the second trimester of pregnancy - ranging from 14 to 22 weeks of gestation - collected from clinically indicated terminations of pregnancies.

The study published in journal Nature showed that the human fetus had functional dendritic cells - crucial to immunity - at 13 weeks gestation.

However the cells' response to foreign proteins differ to adults.

"Similar to adult dendritic cells, fetal dendritic cells migrate to lymph nodes and respond to toll-like receptor ligation; however, they differ markedly in their response to allogeneic antigens," the authors wrote.

That is, they suppress or dampen an immune reaction to cells from the mother.

"Our results reveal a previously unappreciated role of dendritic cells within the developing fetus and indicate that they mediate homeostatic immune-suppressive responses during gestation," wrote the authors.

The result challenged the assumption that the human fetus is unable to mount an immune response.

A developing fetus is constantly exposed to foreign proteins and cells, which are transferred from the mother through the placenta.

Fully understanding that development could reveal the reasons for some miscarriages, as well as explain conditions such as pre-eclampsia, which is associated with abnormal immune responses to pregnancy, says immunologist Mike McCune at the University of California.

"It's important for us to understand the function of the human fetal immune system so that we can treat fetuses that are not doing well," he told Nature.com.