‘Layered immunity’ and the ‘neonatal window of opportunity’ – timed succession of non‐redundant phases to establish mucosal host–microbial homeostasis after birth

The intricate host-microbial interaction and the overwhelming complexity of the mucosal immune system in the adult host raise the question of how this system is initially established. Here, we propose the implementation of the concept of the 'postnatal window of opportunity' into the model of a 'layered immunity' to explain how the newborn's mucosal immune system matures and how host-microbial immune homeostasis is established after birth. We outline the concept of a timed succession of non-redundant phases during postnatal immune development and discuss the possible influence of external factors and conditions.