Ever since the late 20th century, it has been accepted that the lactating human mammary gland elaborates a complex immunological system uniquely adapted to protect the recipient infant. It is not well appreciated, however, that many basic discoveries concerning these matters were made long before the era of molecular biology. Observations during the eighteenth and nineteenth centuries in Europe suggested that mortality was lower in breastfed than in non-breastfed infants. The basis of this protection was unknown. Although similar but more structured observations were made in the first part of the twentieth century (Westergaard, 1901; Groth and Hahn, 1910; Davis, 1913; Woodbury, 1922, 1925; Gruleeet al.1934, 1935) there was little scientific information concerning the immunology of human milk or of milk obtained from other species. This was partly because most of the immune system was not uncovered until the latter part of the twentieth century. However, discoveries concerning the protective properties of milk from humans and other mammalian species that were made during the nineteenth and early twentieth centuries provided the foundation for the molecular research that came thereafter. Furthermore, it is striking that these discoveries were made by individuals eminent in the history of immunology and medicine. Then in the mid-twentieth century, critical information was that led to the paradigm of an immune system in human milk and to the reslization that the system comprises not only antimicrobial factors but also anti-inflammatory and immunomodulating agents. Parallel investigations indicated that immune systems were in other mammalian milks. Because of many differences in the immune systems were in other from the various mammalian species, the scope of the information in this review will be limited to limited to key discoveries concerning the immunological aspects of human milk.