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Immunity of the newborn

Ilan Dalal, MD
Chaim M Roifman, MD, FRCPC, FCACB
Section Editor
E Richard Stiehm, MD
Deputy Editor
Elizabeth TePas, MD, MS


The fetal immune system develops in a sterile and protected environment and therefore lacks antigenic experience. It is also modulated in order to coexist with the mother's immune system. Soon after birth, the newborn is exposed to the "hostile world" of bacteria, viruses, fungi, and parasites and must immediately defend itself. The immunologic competence of the neonate progresses rapidly in the first three months of life as the cells involved in acquired immunity mature and gain antigenic experience. During this period, the neonate mainly depends upon components of the innate or antigen-independent immune system, including phagocytes, natural killer (NK) cells, antigen-presenting cells (APCs), humoral mediators of inflammation, and complement [1-3]. The breastfed infant also receives antimicrobial components in breast milk that help prevent certain acute infections. (See "Infant benefits of breastfeeding", section on 'Anti-microbial components' and "Infant benefits of breastfeeding", section on 'Prevention of illnesses while breastfeeding'.)

The overall performance of the immune system in the neonatal period is diminished in several important respects. As a result, very young infants are more susceptible than older infants to serious bacterial infection, as well as some viral and fungal infections.

Specifically, organisms that can cause significant morbidity in neonates include group B streptococci, Escherichia coli, herpes simplex virus (HSV), cytomegalovirus (CMV), varicella-zoster virus (VZV), respiratory syncytial virus (RSV), and Candida species [1]. These specific organisms cause neonatal infections more frequently than other organisms because the newborn has an increased risk of exposure to these pathogens, not because these organisms have special virulence determinants that are unique.

This review summarizes the knowledge about the function of the different arms of neonatal immunity, including what is known about neonatal defense against the pathogens mentioned above. More comprehensive reviews are available elsewhere [4-7]. Additional information about the development of the cellular components of the immune system is provided separately. (See "The development of immune cells in the fetus and neonate".)

Certain clinical scenarios, such as recurrent severe infections, chronic diarrhea, and failure to thrive, should prompt immediate evaluation for immunodeficiency states. The presentation of these disorders is presented in more detail separately. (See "Severe combined immunodeficiency (SCID): An overview" and "Recognition of immunodeficiency in the newborn period".)


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