Breast-feeding and protection against infection

Microbial exposure and the immune system of the newborn The newborn starts to be colonized in the upper respiratory tract and gastrointestinal tract, as well on the skin, directly from birth onwards. Delivery next to the anus exposes the newborn to the mother’s intestinal microflora, in which harmless anaerobes are predominant, but where aerobic and facultative anaerobes are also prevalent. Some of the latter may contain species that are potentially pathogenic. Before the strict anaerobes have totally taken over, reaching levels over 99%, there is an early period where they have not yet reached numbers high enough to compete successfully with the aerobes and facultative anaerobes for space and nutrients and bring their numbers down. The potential pathogens may then reach levels at which they can initiate infections. Thereafter, the infant is also at a special risk of infections because of its somewhat limited host defences. This is strikingly illustrated by the high early infant mortality caused by infections, as seen in poor regions across the world (1, 2). The infant has a complete immune system at delivery, but it is very small. It will start to expand as soon as it meets all the microbes on the mucosal membranes, especially in the gut. The neonate remains deficient in several defensive functions during the first weeks and months of life (3). This includes, in particular, various aspects of phagocytes and their functions, but also reduced antigen presentation, a slow start to the production of secretory immunoglobulin A (SIgA), which normally protects mucosal membranes where most infections start, and a somewhat reduced capacity of cell-mediated immunity.