Formula, which must often be mixed with water and placed in bottles, can become contaminated easily. Yet even infants who receive sterilized formula suffer from more meningitis and infection of the gut, ear, respiratory tract and urinary tract than do breast-fed youngsters. The reason, it turns out, is that mother's milk actively helps newborns avoid disease in a variety of ways. Such assistance is particularly beneficial during the first few months of life, when an infant often cannot mount an effective immune response against foreign organisms. Indeed, a child's immune response does not reach its full strength until age five or so.
All human babies receive some coverage in advance of birth.
During pregnancy, the mother passes antibodies to her fetus through the placenta. These proteins circulate in the infant's blood for weeks to months after birth, neutralizing microbes or marking them for destruction by phagocytes-immune cells that consume and break down bacteria, viruses and cellular debris. But breast-fed infants gain extra protection from antibodies, other proteins and immune cells in human milk. Once ingested, these molecules and cells help to prevent microorganisms from penetrating the body's tissues. In this way, they block microbes from attaching to and crossing through the mucosa-the layer of cells, also known as the epithelium, that lines the digestive tract and other body cavities.