Breastfeeding is acknowledged by health professionals as the ideal source of nutrition for infants in almost every case. In addition, the nurturing relationship engendered by breastfeeding contributes to building a close bond between mother and child. Together, these factors promote the physical and emotional growth and development of the infant.
Long known to be true, too, is that breast-fed babies experience fewer and less serious incidences of disease and allergy than formula-fed babies. Gastrointestinal, respiratory, and middle-ear infections, in particular, are greatly reduced in breast-fed infants. In recent years, scientists at Penn and other institutions have sought to understand what it is about breast milk that makes it so protective for infants.
"What researchers have discovered is that breast milk is much more than just food," says Charles V. Clevenger, MD, PhD, an assistant professor of pathology and laboratory medicine. "It's also a bioactive compound containing antibodies that defend against infection and hormones and growth factors that direct the infant's immune system to develop fully and appropriately."
For example, Clevenger's studies have shown that the hormone prolactin, which is responsible for growth and differentiation of the breast during puberty, pregnancy, and lactation, also plays a significant role in stimulating immune-system cells in the infant. And while prolactin is primarily produced in the pituitary gland, Clevenger's team found that tissues in the breast are able to make the hormone, too.
Clevenger notes that, in the adult stomach, the proteins that constitute the antibodies, hormones, and growth factors in breast milk would not survive exposure to the digestive acids.
"But infants are different from adults, and one of the ways they are different is that their stomachs don't make as much acid," Clevenger says. "As a result, many of the important immunostimulatory proteins in breast milk can pass through the stomach of an infant and into the intestine, from which they enter the infant's bloodstream largely intact. In the adult, these proteins would be destroyed."
Investigations by other scientists have demonstrated that vaccine responses in breast-fed infants are enhanced over those of formula-fed babies. One proposed mechanism to explain this observation is that certain antibodies in breast milk may mimic bacterial and viral proteins. Such mimicry would have the effect of priming the infant immune system to respond more vigorously when later exposed to actual pathogen proteins.
A number of studies over the last decade have also revealed the presence of more than a dozen cytokines in breast milk. Cytokines are members of a family of proteins secreted by various cells that stimulate, inhibit, or otherwise regulate immune-system cells. Many of these may be produced by T cells in breast milk itself, while others are made by cells within the breast.
Breast milk has very high concentrations of complex carbohydrates, and some of these have been shown to have immune properties, too. Oligosaccharides and glycoconjugates, in particular, may inhibit infection by binding to pathogens at the molecular sites that they would otherwise use to attach to and then attack cells in the infant. Another molecule with an antibacterial role is lactoferrin, a protein that acts by binding iron in different situations. In some circumstances, lactoferrin is also able to act as a cytokine.
Formula-fed babies are denied many of the immune and other benefits that accompany breastfeeding, according to physicians and scientists. Although there are times when formula is preferable to breastfeeding -- such as when the mother is HIV-positive but the child is not -- such cases are rare, they say. In the great majority of situations, mothers should be strongly encouraged by their caregivers to breastfeed their infants and given the necessary support to enable them to succeed.
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