Some T cells may be a fetus' best friend.

During a healthy pregnancy, a woman's immune system somehow manages to avoid attacking the fetus she's carrying, even though it has plenty of foreign characteristics contributed by the father.

In the March Nature Immunology, researchers offer an explanation in experimental mice for this pivotal exercise of self-control: While pregnant, the animals overproduce a kind of T cell that reins in other immune cells that might target the fetus.

Immunologist Alexander G. Betz of the Medical Research Council Laboratory of Molecular Biology in Cambridge, England, and his colleagues found that healthy, pregnant mice have double to triple the number of CD4+CD25+ T cells, also called regulatory T cells, in their blood, spleen, and lymph tissue as do female mite that aren't pregnant.

To test the cells' effect on pregnancy, the researchers mated 30 female mice with males. Half of the females had fully functioning immune systems; the others lacked the regulatory T cells. Nine of the healthy females became pregnant--a percentage slightly higher than normal. None of the female lab mice lacking the T cells could sustain a pregnancy.

Like mice, people harbor a population of the regulatory T cells. Their role in pregnancy remains unclear, but Betz suggests that doctors might look for unusual concentrations of T cells in women who are infertile with no known cause, which is the case in 10 percent to 15 percent of infertile women.

An understanding of the role of regulatory T cells might also lead to new treatments for suppressing rejection of transplanted organs and inhibiting autoimmune reactions, in which a person's immune cells attack his or her own tissues, Betz and his coworkers speculate.