Immunologic aspects of breast feeding, antiinfectious activity of breast milk.
The most apparent immunologic role of the mammary gland is supply to antibodies to the neonate. In cattle the gland must be able to secrete large quantities of IgG antibodies over a short time period to supply the offspring with protection against systemic pathogens. This is accomplished by selective transfer of IgG from serum to the gland followed by eventual absorption by the neonate gut. In all mammals, the mammary glands provide IgA antibodies specific for pathogens or antigens which enter or invade the neonatal gut. An entero-mammary cell circulation provides the mechanism for conveying such specificity to the lacteal IgA antibodies. Some IgA antibodies may also be derived from the circulation so that the quantitative significance of serum derived versus locally produced IgA in different species requires clarifications. IgG and IgG lacteal antibodies ingested by the neonate, provide short-term systemic and long-term enteric humoral immunity to the neonate. In addition to providing passive immunity, at least swine IgG appears to have a regulatory role in the development of the systemic humoral immune system of the neonate. Such a phenomenon may be general for IgG antibodies transferred in colostrum or in utero. While passive antibodies and immunoglobulins may be most important for the neonate, the many other potentially anti-infectious elements transferred in colostrum and milk may also play important roles. 'Bifidus' factor particularly, but also lysozyme and lactoferrin are probably all important although more convincing experimental data will be needed to support this assumption. Studies of cells of the lymphoid and reticuloendothelial systems in milk are more recent and their role in the neonate remains to be convincingly demonstrated. In summary, the immunologic and anti-infectious roles of the mammary glands are (1) Supply of IgA antibodies against enteric antigens to the neonate on a 'long-term' basis throughout lactation; (2) Short-term supply of IgG (and IgA) in Group II and III mammals for eventual absorption into neonatal serum; (3) The supply of numerous nonspecific factors such as 'bifidus factor,' lactoferrin, and lysozyme throughout lactation; (4) Regulation of the development of humoral immunity by an apparent feedback mechanism involving maternal IgG; (5) Self-protection of the gland by sensitized T-lymphocytes acting directly or using lymphokines on macrophages; and (6) Self-protection of the gland by secreted antibodies that may act in complement-independent cytolysis, as opsoins for polymorphonuclear-leukocytes or directly as agents preventing colonization.