The immune system is designed to attack anything that is not the body's own tissues, such as pathogens and genetically nonidentical organ transplants, so why does the maternal immune system not attack a developing fetus? Several answers to this question are provided by a new study of mice from researchers at New York University School of Medicine.

In the study, which appears online on in advance of publication in the May print issue of the Journal of Clinical Investigation, Adrian Erlebacher and colleagues show that when maternal immune cells known as T cells interact with fetal cells they can't "see" proteins that only their fetus expresses. By contrast, the same maternal T cells were able to "see" the fetal proteins when other maternal immune cells began picking up the fetal proteins around mid-gestation.

However, this did not result in the T cells being primed to attack the fetus, rather, it induced the T cells to die. Surprisingly, even when the T cells were isolated from the female mice and exposed to the fetal proteins in vitro, under conditions that normally stimulate T cell activation, the maternal T cells did not become activated. This study therefore describes three reasons why maternal T cells do not attack a developing fetus and the authors suggest that immune-mediated early pregnancy loss might occur if maternal T cells become able to "see" fetal proteins when they interact with fetal cells.