M cells at locations outside the gut

Lymphoid tissue associated with mucosal membranes is found not only along t he gastrointestinal tract, but also in the tonsils, the upper and lower air ways, and the conjunctiva of the eye. The epithelia overlying this mucosa-a ssociated lymphoid tissue (MALT) contain membranous (M) cells which transpo rt antigenic matter across the mucosal membrane to initiate immune response s. Although the morphology and function of intestinal M cells have been tho roughly studied relatively little is known about the presence and propertie s of M cells in MALT outside the gut. The available data on ultrastructure, histochemistry, and antigen sampling function of the epithelia in tonsils, nasal-, larynx-, bronchus-, and conjunctiva-associated lymphoid tissue are reviewed and critically discussed. It is concluded that, in principle, the concepts of mucosal immune protection can De applied to these sites of MAL T. However, it is questionable whether a separate cell type similar to inte stinal M cells exists and performs antigen sampling in the different MALT e pithelia. Further studies combining functional and morphological techniques are essential to understand the initiation of immune reaction at the mucos al membranes.