Lactating mammary glands are part of an integrated mucosal immune system with local production of antibodies, mainly consisting of secretory immunoglobulin A (SIgA). These antibodies generally reflect antigenic stimulation of mucosa-associated lymphoid tissue (MALT) by common intestinal and respiratory pathogens. Antibodies in breast milk are thus highly targeted against infectious agents in the mother's environment, which are those likely to be encountered by the infant shortly after birth. Therefore, breast-feeding represents an ingenious immunological integration of mother and child. The mucosae are favored as portals of entry by most infectious agents, and the neonatal period is particularly critical in this respect. Mucosal pathogens are a major killer of children below the age of 5 years, being responsible for more than 14 million deaths annually. Diarrheal disease alone claims a toll of 5 million children per year in the developing countries. Epidemiological data suggest that the risk of dying from diarrhea could be reduced 14-24 times in breast-fed children. A beneficial clinical effect is also apparent in the industrialized world, even in relation to relatively common diseases such as otitis media and acute lower respiratory tract infections.