The human immune system develops in waves, the first of which begins even before birth. Fetal and adult T cells originate from different stem-cell populations, allowing the fetal immune system to better tolerate foreign antigens — namely the mother's.

Joseph McCune at the University of California, San Francisco, and his colleagues compared human fetal blood stem cells and T cells with those of adults. After implantation in mice that permit human blood-cell maturation, fetal stem cells were more likely than adult ones to develop into regulatory T cells. These suppress immune activity, enhancing tolerance to antigens.

Fetal stem cells and T cells also had different gene-expression profiles from the adult versions of these cells. Statistical analysis revealed that developmental stage accounted for most of these differences.

Science 330, 1695–1699 (2010)