Human milk contains a wide variety of proteins that contribute to its unique qualities. Many of these proteins are digested and provide a well-balanced source of amino acids to rapidly growing infants. Some proteins, such as bile salt-stimulated lipase, amylase, beta-casein, lactoferrin, haptocorrin, and alpha1-antitrypsin, assist in the digestion and utilization of micronutrients and macronutrients from the milk. Several proteins with antimicrobial activity, such as immunoglobulins, kappa-casein, lysozyme, lactoferrin, haptocorrin, alpha-lactalbumin, and lactoperoxidase, are relatively resistant against proteolysis in the gastrointestinal tract and may, in intact or partially digested form, contribute to the defense of breastfed infants against pathogenic bacteria and viruses. Prebiotic activity, such as the promotion of the growth of beneficial bacteria such as Lactobacilli and Bifidobacteria, may also be provided by human milk proteins. This type of activity can limit the growth of several pathogens by decreasing intestinal pH. Some proteins and peptides have immunomodulatory activities (eg, cytokines and lactoferrin), whereas others (eg, insulin-like growth factor, epidermal growth factor, and lactoferrin) are likely to be involved in the development of the intestinal mucosa and other organs of newborns. In combination, breast-milk proteins assist in providing adequate nutrition to breastfed infants while simultaneously aiding in the defense against infection and facilitating optimal development of important physiologic functions in newborns.