Читать книгу Neonatal Haematology - Barbara J. Bain, Irene Roberts - Страница 34

The clinical importance of the immaturity of the innate immune system in neonates is best demonstrated by the fact that the pattern of infections in neonates closely mimics that seen in children with SCN. While neutrophil numbers at birth are similar to those in older children and adults, the lack of neutrophil reserves discussed above is aggravated by the impaired function of neonatal neutrophils.⁹⁵ First, recruitment and migration of neonatal neutrophils to sites of infection is impaired compared with adult neutrophils. The processes involved are complex and several aspects are defective in neonatal neutrophils, including chemotaxis, adhesion, rolling and transmigration, which together result in an impaired ability to leave the circulation and enter the tissues.^120–122 Secondly, neonatal neutrophils have an overall reduced ability to generate neutrophil extracellular traps (NETs), which are an important component of the innate immune response that limits the dissemination of a variety of pathogens.^123–125 NETs are extracellular, web‐like structures composed of a variety of antimicrobial molecules, such as elastase, myeloperoxidase, lactoferrin and defensins, and are key for protection against infection in neonates, trapping, neutralising and killing bacteria, fungi, viruses and parasites.¹²⁶ Thirdly, neonatal neutrophils have lower levels of various antimicrobial granule proteins, such as lactoferrin and bactericidal/permeability‐increasing protein (BPI), particularly in preterm neonates.⁹⁵ Recent data suggest that lactoferrin may be particularly important for converting neonatal neutrophils and monocytes to myeloid‐derived suppressor cells (MDSC), which are now recognised as being critical in controlling diseases associated with deregulated inflammation in neonates, including NEC.^127,128 Finally, although term neonates are able to phagocytose both Gram‐positive and Gram‐negative bacteria normally, in preterm neonates phagocytosis of bacteria and of Candida albicans is less efficient.^102,129