Immune development in early life.

The perinatal period of immune ontogeny reflects prenatal, maternal and postnatal cues that converge to establish durable immune trajectories. Fetal and infant immunity follows distinct, ontogeny-specific programs optimized for tissue protection, tolerance and rapid effector function. Herein, we integrate recent insights into prenatal hematopoiesis, tissue seeding by innate and adaptive lymphocytes, and the establishment of immune specialization across barriers and systemic compartments. We examine how cellular and humoral factors in blood, tissues and/or breast milk calibrate immune development, and how postnatal microbial colonization and environmental exposures further refine immune function in a tissue- and time-dependent manner. Together, these findings redefine early life as a critical window during which immune set points are established, with lasting immunological imprinting linked to host defense, vaccine responsiveness and risk for atopy. A mechanistic understanding of these processes provides a framework for rational, age-adapted strategies to promote immune health across the life course.