Protective factors in milk and the development of the immune system.

The neonate is immature in certain immunologic functions. The slow development of secretory immunoglobin A (IgA) seems to be compensated by selective transfer of secretory IgM into exocrine secretions on mucous membranes during the first few months of life. Secretory IgA and secretory IgM antibodies against Escherichia coli and poliovirus are already found in the neonate, possibly in response to the maternal anti-idiotypic IgG antibodies transplacentally exposing the fetus. Via such a mechanism, food antibodies could occur before direct food exposure in the infant. Human milk provides large amounts of antibodies (as a crude comparison, about 50 times the amount of antibodies given to a patient with hypogammaglobulinemia). The milk antibodies, dominated by secretory IgA, protect especially against intestinal infections. The milk also contains oligosaccharide analogues to epithelial receptors for bacteria. They, as well as a number of milk components such as lactoferrin and lysozyme, may contribute to host defense. The food antibodies in human milk may influence the infant's immune response to foreign food proteins introduced during weaning.