Current Topics in Host Defenses of the Newborn

Abstract

Growing recognition of the cellular and humoral constituents present in human milk has led to a reexamination of the role of milk in host defenses. Three basic types of materials are present. (1) Cells, including B cells, T cells, and macrophages. Current data suggest that the lymphocyte populations may be unique to the gut-mammary circulation and offer a selective advantage against such organisms as K1 positive E. coli. In the rat, such cells have been shown to persist for periods up to 2 weeks in the stomach of suckling newborns. If this observation is also true of the human neonate, then the potential for significant immune function resulting from milk lymphocytes exists. Milk macrophages may play an important role in protecting the newborn against certain forms of necrotizing enterocolitis. (2) Immunoglobulins. In general, colostral and breast milk antibodies do not contribute to the pool of serum immunoglobulins. Antibody titers of colostrum and serum may differ markedly. The role of milk antibodies in neonatal host defenses is unclear. Presumably, they exhibit protective activity against a range of enteric pathogens, but their precise role remains to be defined. (3) A variety of “nonspecific” humoral factors. These include lysozyme, lactoferrin, lactoperoxidase, an antistaphylococcal fatty acid and, perhaps, active complement components. Although the synthesis of this new information into a clear description of how human milk modulates host defenses is incomplete, it seems probable that an important biologic advantage is provided to the nursing infant.