Fig. 1.

Breast milk is immunologically active at the infant’s intestinal mucosa. Microbicidal factors include activated lymphocytes and phagocytes, secretory IgA and IgM, complement and other antimicrobial peptides and lipids, all of which provide broad immune activity against gut pathogens. Maternal IgG from cross-placental transfer provides further support, and the antimicrobial properties of breast milk are complemented by an anti-inflammatory activity, which downregulates damaging inflammation within the mucosa. Direct immunomodulatory factors include cytokines and growth factors, some of which interact directly with the mucosal epithelium, and others that have more distant targets (IL-7 targets the thymus – not shown). Pre-biotic factors including acidic and neutral oligosaccharides facilitate the growth of a healthy intestinal microbiome, and crosstalk between gut commensals, mucosal immune tissue (shown in yellow), and immunomodulators in breast milk help to drive tolerance to harmless antigens.