Why are infants more susceptible to infectious diseases and less responsive to vaccines ?

Arnaud Marchant is one of the few immunologists to have specialized in the analysis of the development of the immune response in newborns and infants. He has shown us that, contrary to popular belief, the very young child is not, strictly speaking, immunocompromised. His or her B, T and dendritic immune cells have undergone a maternal imprint during gestation, which involves a certain degree of immaturity compared with that of the older child and adult, but which in reality corresponds to the unfolding of an extremely complex and sophisticated program of evolutionary changes in the various parameters of the immune response, and in particular to the constraints of the period of exposure to new environments, including the microbial environment and the mother's milk diet.