Very little is known about the timing for the T cell development in infants and young children that, in sharp contrast with mice, lasts until puberty. Our work suggests that immune regulation is the main efferent arm of the innate and adaptive immunity in pediatric age. Maternal immune cells are transferred with the milk to the baby indicating that the immunological microchimera plays an important protective role in the immune homeostasis of infants. Here we review the immune cells of the mother/offspring microchimera in the uterus/placenta interface during pregnancy and the immune cells transferred to the baby by maternal milk after birth. We also discuss possible outcomes that may lead to pathology with a new light on T cell alloreactivity.