In the gastrointestinal tract, the mucosal surfaces facing the lumen form a highly integrated network of host cells and factors essential to preserve the tight barrier keeping in control the translocation of various infectious and non-pathogenic microorganisms. Epithelial cells, the gut-associated lymphoid tissue, immunoreactive cells distributed within the mucosal tissue, and antibodies act in concert to promote induction of an immunological barrier comprising innate and adaptive components. This physiological interface senses continuously its ever-changing environment through selective uptake of luminal antigens by microfold (M) cells present at the apex of Peyer’s patches spread in the follicle-associated epithelium overlying the sites of initiation of mucosal immune responses. Depending on the nature of the sampled antigens, discriminating responses will be elicited, ranging from tolerance, immune suppression, inflammation, or specific adaptive reactions.
In this process, the local production of a specific type of antibody referred to as secretory IgA (SIgA) is instrumental to the maintenance of mucosal homeostasis. SIgA serves as a first line of defense against various antigens by preventing their interaction with the epithelium, and thus leading to their elimination by peristalsis. In addition, SIgA-based immune complexes actively contribute to the sensing of noxious antigens under non-inflammatory conditions, thus ensuring recognition in the absence of undue inflammation. Furthermore, SIgA modulates the intensity of induced immune responses, a process involving targeting of dendritic cells in the Peyer’s patch. The role of SIgA in controlling the commensal microbiota is another facet of the multiple mechanisms by which the antibody fulfills its complex function at mucosal surfaces.