The mother transmits immune factors to the offspring both through the placenta and milk. Large quantities of IgG are transmitted via the placenta, whereas other immunoglobulin isotypes are not. Consequently, virtually all IgG in neonatal blood is of maternal origin, the concentration of IgG in umbilical cord blood is somewhat higher than in adults, and the levels of other immunoglobulin isotypes are exceptionally low. Low concentrations of some factors such as IgG and secretory IgA antibodies are also transmitted via amniotic fluid, but little is known about their in vivo effects upon the fetal mucosal immune system.
An array of host resistance factors are transmitted to the infant in human milk, including leukocytes, secretory IgA, lactoferrin, lysozyme, and oligosaccharides and glycoconjugates that are receptor analogs for microbial adhesins and toxins. In addition to those antimicrobial factors, anti-inflammatory agents, and immunomodulating agents including TNF-α, TGF-β, IL-1β, IL-6, IL-8, IL-10, G-CSF, and M-CSF. These factors are designed to act at mucosal sites and to protect by noninflammatory mechanisms. Since the endogenous production of these agents is incompletely developed in early infancy and they are scarce in cow's milk or other substitute feedings, it is not surprising that breastfeeding increases resistance to gastrointestinal and respiratory infections, allergic diseases, and certain inflammatory diseases that occur much later in childhood.