Oligosaccharides and glycoconjugates in human milk: their role in host defense

Human milk contains an extremely high concentration of complex carbohydrates, especially oligosaccharides, the third most abundant solid constituent of human milk. The value of human milk nutrients to infants is now widely recognized, and a role for the secretory antibodies of human milk in the defense of the infant is generally accepted. However, a function for nonimmunoglobulin milk protective factors, many of them non-nutrients, in providing for the defense of the nursling is only now beginning to be appreciated. Prominent among postulated defense agents are the milk oligosaccharides and glycoconjugates. Their complex carbohydrate structures are thought to be assembled by the same enzymes, the glycosyltransferases, that synthesize the cell surface glycoconjugates often used as receptors by pathogens. Some milk oligosaccharides and glycoconjugates may protect the nursing infant by acting as receptor homologs, inhibiting the binding of enteropathogens to their host receptors. Ongoing research is linking specific carbohydrate structures with protection against specific pathogens. Current information regarding the composition, protective activities, and protective mechanisms of the milk glycolipids, glycoproteins, mucins, glycosaminoglycans, and oligosaccharides is reviewed.