The transfer of host defence capacity to the human offspring provides a remarkable model of passive transfer of immunity. In fact it may also provide an example of active immunization. The transfer of mucosal protection via breast feeding offers many additional advantages for the mother and infant. Through its contraceptive effects it increases the spacing between births, thus diminshing the infant mortality and the burden on the mother. It also enhances bonding between mother and child, it seems to increase the IQ and school result of the infant and might decrease the risk of certain malignancies and perhaps of juvenile diabetes. A fully breast-fed infant receives as much as 0.5-1 g of secretory immunoglobulin A (SIgA) antibodies daily, the predominant antibody of human milk. This can be compared to the production of some 2.5 g of SIgA per day for a 60 kg adult. These SIgA antibodies have been shown to protect against Vibrio cholerae, ETEC, Campylobacter, Shigella and Giardia. Furthermore, milk is rich in receptor analogues for certain epithelial structures which microbes need for attachment to host tissues as an initial step in infections. Thus the adherence of Haemophilus influenzae and pneumococci for example to retropharyngeal cells is efficiently inhibited by human milk. This may be one explanation for the fact that breast-fed babies have less otitis media than the non-breast-fed. Other milk factors like lysozyme and lactoferin may contribute to the host defence, but this has not yet been well defined. However, human milk also supports the well-being of the infant by being anti-inflammatory.(ABSTRACT TRUNCATED AT 250 WORDS)