directed against pathogens encountered in the environment that are common to both the mother and infant (see box above) (Goldman and Goldblum, 1989b). These maternal antibodies are of particular importance because the secretory immune system of the infant does not mature for several months after birth (Burgio et al., 1980; Hanson et al., 1983).

The main antibody in human milk is secretory IgA (dimeric IgA coupled to secretory component) Butte et al., 1984a; Goldman and Goldblum, 1989b; Goldman et al., 1982, 1983a,b). IgM and IgG are also found in human milk but in much lower concentrations (Goldman and Goldblum, 1989a). The IgA-producing cells in the mammary gland tissues originate from B cells from either the small intestine or the respiratory tract and enter the systemic circulation. Then, lactogenic hormones stimulate the B cells to travel to the mammary gland (Weisz-Carrington et al., 1978), where they are transformed to plasma cells that produce dimeric IgA. Because these B cells originate at maternal sites where exposure to environmental pathogens is high, the IgA is protective against pathogens to which the infant might be exposed.

Secretory IgA has at least three other important features: it is particularly suited to act at mucosal surfaces, since it is relatively resistant to proteolysis (Lindh, 1975); it protects by noninflammatory mechanisms (Goldman et al., 1986, 1990); and it acts in synergy with several other host resistance agents in human milk to achieve antimicrobial effects.

Lysozyme is a protein in human milk that affords protection in two different ways: it breaks down susceptible bacteria by cleaving peptidoglycans from their cell walls (Chipman and Sharon, 1969), and it acts in concert with other