Host resistance factors in human milk.

PIP: This paper discusses the nature of host resistance factors in human milk and epidemiologic studies regarding infections and mortality rates in breastfed and nonbreastfed babies. The defense factors and their proposed modes of action are: 1) a growth enhancer of lactobacilli, which interferes with intestinal colonization of enteric pathogens; 2) antistaphylococcal factors, which inhibit staphylococci; 3) secretory IgA and other immunoglobulins, which protect the gut and respiratory tract; 4) C4 and C3 (complement components; C3 fragments have opsonic, chemotactic, and anaphylatoxic activities); 5) lysozome, lysis of bacterial cell wall; 6) lactoperoxidase, killing of streptococci; 7) lactoferrin, kills microorganism by chelating iron, and 8) macrophages and lymphocytes, phagocytosis and cell-mediated immunity. Although it can be postulated that the breastfed infant's resistance to infection would be superior on account of the greater presence of these factors in human milk compared to cow's milk, little is known about the effects of these defense factors on the infant. Epidemiologic studies have reported on the lower morbidity and mortality rates of breastfed infants as compared to bottlefed infants. Other studies have focused on the protective effects of human milk upon the infant, but these have been inconclusive. In countries with poor sanitation and high infection rates, the incidence of bacterial infections is lowest in breastfed infants. The advantages of human milk however are difficult to demonstrate in societies with high standards of sanitation and low infection rates. Infection and mortality rates in infants have in fact declined in developed countries as the practice of breastfeeding declined. Until it is established that immunity to common pathogens is transmitted to the infant by human milk, it will not be known whether human milk does have protective effects.^ieng