Immunology of Breast Milk

A most important adaptation of the gastrointestinal (GI) tract of the newborn to the extrauterine environment is the development of a mucosal barrier against the pénétration of bacteria, viruses, and other environmental antigens, i.e., food proteins. The mechanisms of host défense at mucosal sites, both immunologie and nonimmunologic, are underdeveloped during the neonatal period and more so in prématurés and small for date neonates (1). The intestinal epithelial cells lack a well-defined brush border and retain a primitive transport mechanism for endocytosis of large molécules (2). It has been proven that the neonatal intestine may absorb larger quantities of ingested antigenic proteins and proliferating microorganisms than the adult intestine (3). The number of the Peyer's patches is small in mammals during the neonatal period and although récent studies hâve described T-helper and T-suppressor lymphocytes, B-lymphocytes, and macrophages on their lymphoid follicles (4), the functional ability of the cells seems to be immature until about 1 month of âge (5). Furthermore, plasma cells, which are normally abundant in the mucosal lamina propria, are not detected in utero or during the immédiate neonatal period (6). Taking into considération ail of thèse immaturities and the absence of mucosal antigenic stimulation during intrauterine life, it is no wonder that human and other mammalian neonates exhibit characteristic absence of secretory IgA (slgA) during the early postnatal period. Shortly after birth, however, the ingestion of food antigens and the colonization of the gut with various microorganisms provide enough antigenic stimulation to evoke a secretory immune response. By 1 month of âge, approximately ail infants will hâve some détectable slgA (7). The availability of immunological factors in the sécrétions of the mammary gland may represent an evolutionary event to compensate through breast feeding for the transient deficiency of mucosal immunity in the neonate. Indeed human colostrum and milk bear many similarities to the external body fluids bathing the mucosal surfaces of the GI and respiratory tracts. Evidence so far also suggests that the immunoglobulin-producing cells observed in the mammary tissue during lactogenesis and in the colostrum and milk are of mucosal origin (8). Fresh human milk contains many components that provide spécifie as well as nonspecific défenses against infectious agents or other macromolecules (9-11).