M cells are higly specialized cells within the follicle-associated epi
thelium (FAE) of the respiratory and intestinal tracts. They play a ce
ntral role in the induction of the mucosal immune response by transpor
ting antigens to the lymphoid tissue. In this way the immune system ma
y encounter the limitless variety of antigens that enter the body thro
ugh the gut and airways. Here we describe the structure and the functi
on of intestinal M cells. In addition, controversial issues relating t
o the M cell biology, such as APC function and the origin of M cells w
ithin the FAE, are discussed.