Mucosal epithelia have two functions, which at first glance seem at variance with each other — maintenance of a barrier and uptake of antigens. The M cell is a unique solution to this problem. However, it also provides many mucosal and systemic pathogens with a convenient portal of invasion, and may also be the point of uptake of detrimental antigens, such as carcinogens and those that induce autoimmune disease. Much remains unknown about the functions of M cells and the precise nature of their interactions with pathogens. Determinants of pathogen tropism for M cells, in particular anatomic sites, must be elucidated before we can proceed to the task of modulating M cell antigenic uptake to enhance absorption and presentation of vaccines and to reduce systemic penetration of allergens and pathogenic microorganisms. Research in this area would be considerably advanced by the development of techniques to induce and maintain differentiated M cells in vitro, and the production of monoclonal antibodies specific for cells of this lineage. Irrespective of these gaps in our current knowledge, it has become increasingly apparent that the M cell may be of paramount importance in the initiation of colonization by many, if not all, important human and animal mucosal pathogens.
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