M cells at locations outside the gut

Lymphoid tissue associated with mucosal membranes is found not only along the gastrointestinal tract, but also in the tonsils, the upper and lower airways, and the conjunctiva of the eye. The epithelia overlying this mucosa-associated lymphoid tissue (MALT) contain membranous (M) cells which transport antigenic matter across the mucosal membrane to initiate immune responses. Although the morphology and function of intestinal M cells have been thoroughly studied, relatively little is known about the presence and properties 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 be applied to these sites of MALT. However, it is questionable whether a separate cell type similar to intestinal M cells exists and performs antigen sampling in the different MALT epithelia. Further studies combining functional and morphological techniques are essential to understand the initiation of immune reaction at the mucosal membranes.