Immune factors in human milk

The lactating mammary gland is a relevant part of the infant immune system since breastfeeding provides the postnatal link that promotes maternalinfant immune dialog [1]. In fact, human milk contains a wide spectrum of viable immune cells and immunoactive components, including proteins, peptides, fatty acids, oligosaccharides, hormones, cytokines, antibodies, soluble receptors for bacterial lipopolysaccharide, and microRNA-containing exosomes (all at physiologically relevant amounts) which remain active once in the infant gastrointestinal (GI) tract and contribute to the immune protection and immune development of the infant’s naive immune system, allowing immune autonomy of the child after weaning [24]. Interestingly, the beneficial effects of breastfeeding continue after weaning and well beyond the lifetime of immunoglobulins (Igs) and milk’s other soluble factors. Recent work has shown that milk not only confers short-term passive immunity against relevant antigens in the motherinfant environment but also a persistence of lactationally transferred immunity to adulthood [5,6], a fact that suggests the existence of a more active mechanism through which transferred cells drive the development of the neonate’s T-cell repertoire [7]. Similarly, murine studies have revealed that lactational transfer of immunity can cross MHC class I barriers and that Th1 immunity can be imparted to Th2-biased offspring [8]