The immune system of preterm infants: an overview.

Every year, approximately 13 million infants are born preterm (<37 weeks gestation). Preterm-born infants experience disproportionately high infection-related morbidity and mortality, reflecting the immaturity of their immune system, especially early in life. This review provides an up-to-date overview of the phenotype and function of the immune system in preterm infants compared with term infants, with an emphasis on adaptive immunity. At birth, both innate and adaptive immune cells of preterm infants show phenotypic and functional immaturity. In addition, antibody levels are reduced, and immunogenicity of some vaccine components is diminished, contributing to impaired pathogen clearance and suboptimal vaccine responses. During the first year of life, rapid maturation occurs and differences with term infants become less pronounced or disappear. This review provides readers with a framework for understanding the immunologic mechanisms underlying the increased infection risk in preterm-born infants. Recognizing the all-encompassing nature of immune immaturity in preterm infants is essential for the development of integrated strategies to further improve health outcomes.