In human milk, there is a complex system of antimicrobial factors (Goldman and Goldblum, 1989b, 1990) (Table 6-4) with the following main characteristics:

• The factors are biochemically heterogeneous.

• Most of the factors are produced throughout lactation.

• The factors are relatively resistant to the digestive processes of the infant's gastrointestinal tract.

• Many of the factors interact in inhibiting or killing microbial pathogens.

• The immunologic factors protect by noninflammatory mechanisms.

• The factors of the system are common to mucosal sites and appear to protect principally the digestive tract and the respiratory system of the infant.

There is an array of moderate-chain-length carbohydrates (oligosaccharides and glucoconjugates) in human milk (Cleary et al., 1983; György et al., 1974; Holmgren et al., 1981; Kobata, 1972; Otnæss and Svennerholm, 1982; Otnæss et al., 1983; Svanborg-Edén et al., 1983). Some of these appear to be protective even though they are present in low concentrations. Nitrogen-containing sugars promote the growth of lactobacilli (György et al., 1974), the dominant bacteria in the lower intestinal tract of breastfed infants (Gyllenberg and Roine, 1957; Smith and Crabb, 1961). These lactobacilli appear to protect against the colonization of bacterial pathogens by secreting inhibitory organic compounds such as acetic acid. Specialized oligosaccharides, including monosialogangliosides and glucoconjugates, inhibit the binding of selected bacterial pathogens or their