Host defenses: development and maternal contributions.

During the intrauterine period, the human immunologic system develops through a complex but orderly series of events. The functional capacity of the system remains incomplete, not only during prenatal life but also through much of infancy. Many of the factors not produced by the fetus or infant are provided by the mother. Systemic immunity is augmented by specific IgG antibodies from the placenta and mucosal immunity by a wide array of defense agents from human milk including sIgA antibodies, lactoferrin, lysozyme, other soluble factors with antimicrobial properties, and specifically adapted leukocytes. It appears that the defense of the infant and the maternal contribution to that defense are geared to protect principally by noninflammatory mechanisms. Although much has been discovered about the ontogeny of the human immunologic system and the maternal contributions to this immunity, much remains to be learned about the molecular controls of the system, the fate of the transported maternal factors, feedback mechanisms between the immunologic systems of the mother and infant and the precise effects of maternal factors upon the infant. Answers to these questions may lead to the development of immunizing agents which are better suited to the infant, mucosal immunogens fashioned to stimulate the production of protective SIgA antibodies in human milk, the provision of defense factors for serious infections in young infants, and ways to enhance the maturation of the immunologic system of the infant when that is desirable.