Structure and function of intestinal M cells.

M cells are structurally distinctive, uniquely permeable epithelial cells found only overlying the domes of mucosal lymphoid follicles. Antigenic macromolecules and some viruses, bacteria, and protozoa enter their apical surface by endocytosis or phagocytosis. These substances traverse the M-cell cytoplasm by transcytosis, breaching the epithelial barrier, and then interact with the subepithelial immunocompetent cells to initiate mucosal and systemic immune responses. The M cell serves as a portal of entry for selected pathogens that cause disease locally in the wall of the intestine or, following dissemination, at distant sites. The mechanisms that regulate adherence to and penetration of M cells by macromolecules and microorganisms are not known, but selective binding of secretory IgA to the luminal surface may be important. Whether M cells simply serve a sieving function and always transport substances unchanged across the epithelial barrier or whether they also sort and process antigens they endocytose and present them to adjacent lymphoid cells requires further study.