Ontogeny of the Pulmonary Immune System

Unique features of the developing immune system contribute to increased infant susceptibility to disease, and external insults during this window of susceptibility have a persistent impact on lung function later in life. Postnatal maturation of systemic immunity involves both innate and adaptive arms of the immune system and does not occur in a linear fashion. Infant innate immunity is marked by attenuated granulocyte and monocyte function and altered Toll-like-receptor signaling, while distinct numbers, phenotypes, and functions of T, B, and NK cell populations characterize infant adaptive immunity. In the lung, seeding of alveolar macrophages and lymphocytes within the airways, expression of pulmonary cytokines and chemokines, and regulation of inflammatory cell trafficking occur both prenatally and postnatally. Alveolar macrophages, dendritic cells, and the airway epithelium in the infant lung display distinctive phenotypic and functional properties compared to adult cells. Together, the unique systemic and mucosal immune systems present during early life define the infant lung's response to infection and environmental insults.