Mucosal immunity reduces the need for elimination of penetrating exogenous antigens by proinflammatory systemic immunity. The adult gut mucosa contains some 80% of the body’s activated B cells—differentiated to plasmablasts and plasma cells (PCs). Most mucosal PCs produce dimeric immunoglobulin A (IgA), which, along with pentameric immunoglobulin M (IgM), can be exported by secretory epithelia expressing the polymeric immunoglobulin receptor. Immune exclusion of antigens is performed mainly by secretory IgA in cooperation with innate defenses, but, in newborns and in IgA deficiency, secretory IgM is important. In the gut, induction and regulation of mucosal immunity occurs primarily in gut-associated lymphoid tissue—particularly the Peyer’s patches—and also in mesenteric lymph nodes. Terminal differentiation to PCs is accomplished in the lamina propria to which the activated memory/effector T and B cells home. Lactating mammary glands are part of the secretory immune system, and IgA antibodies in breast milk reflect antigenic stimulation of gut-associated lymphoid tissue and nasopharynx-associated lymphoid tissue such as the tonsils. Breast-milk antibodies are thus highly targeted against infectious agents and other exogenous antigens in the mother’s environment, which are those likely to be encountered by the infant. Therefore breast-feeding represents an ingenious immunologic integration of mother and child.