Bovine milk is a complex mixture of proteins, carbohydrates, fats, vitamins, minerals and growth factors. Its primary role is to provide nutrition during early neonatal development. Milk also helps to protect the newborn from infectious diseases. This is an important function because, at birth, an infant’s immune system has yet to acquire the ability to protect against infections. Milk achieves protection in two ways: first by the passive transfer of immunological molecules such as antibodies that neutralize specific pathogens. This is particularly true for the colostrum of ruminants, which contains high concentrations of antibodies, thus compensating for the lack of transfer of immunoglobulins across the placenta. Second, milk nurtures and orchestrates the development of the immune system so that responses are effective in dealing with a variety of pathogens while minimizing the risk of damage to the neonate’s own tissues. Thus, apart from supporting the rapid growth of tissues and providing energy to fuel that growth, milk may be viewed as a nutrient that establishes a healthy immune system.