Human milk contains a very large number of specific and non-specific immunologic and nonimmunologic factors who provide passive and active protection to the newborn. The immunologic factors are either immunostimulatory or immunosuppressive. The immunostimulatory ones increase host defense mechanisms mainly against infective illness, and the immunosuppressive ones downregulate inflammation and the development of allergies. Furthermore, human milk has been found to promote intestinal growth and maturation and to have immunomodulating effects beyond infancy, later on in life. In conclusion, human milk represents a very valuable weapon for enhancing the immature immunologic system of the newborn and for strengthening its deficient host defense mechanisms against infective or other foreign agents.