Breast-feeding and its role in early development of the immune system in infants: consequences for health later in life.

Respiratory tract infections and gastrointestinal tract infections of both bacterial and viral origin cause the highest mortality and morbidity in neonates and infants. This is true not only for developing countries but also for industrialized countries (1). Increased susceptibility to infections and decreased immune responsiveness to these infectious agents continue to be present significantly in y 2 of life. However, it should be realized that the induction of an immune response against nonharmful common environmental antigens, such as food antigens and particular commensals (bacteria), has to be inhibited lest it give rise to undesirable, excessive, and destructive inflammatory and allergic reactions (2,3). It appears that the development of the immune system in neonates and young infants is reflected in the enhancement of ‘‘specific’’ immune responses to danger signals and in the induction of tolerance toward common nonharmful environmental antigens such as food components as well as the microbiota of the infant gut. It should be realized that the human immune system can be modulated easily during the first months of life (4), when it can be affected not only positively but, unfortunately, also negatively. This dichotomy is illustrated by, e.g., survival advantage after surgery early in life and survival postsurgery health consequences later on (5). This article provides a brief overview of the current knowledge of the development of the infant immune system and possibilities for intervention and immunomodulation.