The human mammary gland is an integral effector component of the common mucosal immune system. However, from physiological and immunological aspects, it displays several unique features not shared by other mucosal sites. The development, maturation, and activity of the mammary gland exhibits a strong hormonal dependence. Furthermore, in comparison to the intestinal and respiratory tracts, the mammary gland is not colonized by high numbers of bacteria of enormous diversity and does not contain mucosal inductive sites analogous to the intestinal Peyer's patches. Consequently, when exposed to antigens, local or generalized immune responses are low or not present. Comparative evaluations of various immunization routes effective in the induction of antibodies in human milk are limited. Systemic immunization induces IgG antibodies in plasma, but due to the low levels of total IgG in human milk, their protective effect remains unknown. Oral or intranasal immunization or infection induces secretory IgA in milk, as demonstrated in several studies. Other routes of mucosal immunization, such as sublingual or rectal exposure effective in the induction of antibodies in various external secretions, have not been explored in the mammary gland. Because secretory IgA in milk displays protective functions, alternative immunization routes and antigen delivery systems should be explored.
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