M cells (or microfold cells, a name given due to their unique structure) are specialized intestinal epithelial cells that are primarily found overlying GALT lymphoid follicles such as the Peyer’s patches in the ileum). These cells do not possess normal intestinal microvilli, in contrast to normal intestinal enterocytes (epithelial cells), and are specialized to sample macromolecules (antigens and pathogens). M cells function to sample and transport antigens/pathogens from the luminal surface to the sub-epithelium (a process also known as transcytosis), where macrophages and other immune cells process the antigen/pathogens. This is achieved by a unique cellular structure involving many basolateral membrane invaginations that allow for the macrophages and other immune cells to initiate an immune response.
M cells play a vital role in immunity allowing immune responses to occur in response to intestinal pathogens/antigens. It is likely that many, if not all, intestinal antigens are initially processed by M cells. Some note that a decrease in M cells leads to failing immunity, such as in old age or chemotherapeutic states. It has been noted that many pathogens exploit this immune surveillance system, using the M cells as a method to gain access to the bloodstream/other parts of the body. Several intestinal pathogens also specifically target M cells to avoid immune processing.
The unique method of M-cell, trans-epithelial transport is also being explored as an intestinal drug and vaccine delivery option. There is still much research being done involving M cells, and it is likely that in 5 years or less this topic will be further expanded.