Mechanisms of antigen uptake in the small and large intestine: The role ofM cells in the initiation of immune reactions

The gut-associated lymphoid tissues, e. g., the Peyer's patches and the app endix, constantly internalize antigenic material to rapidly generate an imm une response, if necessary. This sampling of antigens is performed by speci alized epithelial cells, the ,,membranous" or ,,microfold" (M) cells of the dome epithelia. M cells possess a unique ultrastructure and are typically in contact with lymphoid cells. They endocytose macromolecules and particle s, including entire microorganisms, at their apical membrane, transport the se in vesicles to their basolateral membrane, and exocytose them to the int ercellular space. This article reviews the structural and functional charac teristics of M cells in the digestive tract in humans and other species. Sp ecializations of M cells for antigen uptake and transport comprise the comp osition of their apical membrane, a modified cytoskeleton as compared to en terocytes, and a large pocket-like invagination of the basolateral membrane populated by lymphocytes. Besides ultrastructural characteristics, histoch emical markers are listed that are available for detecting M cells. The ori gin and differentiation pathways of M cells and enterocytes of the dome epi thelium are outlined and critically commented on. Because M cells are known entry sites of various pathogens and, in the future, might be employed for the oral application of drugs and vaccines, the clinical relevance of M ce lls in health and disease is discussed.