Immunology | Immune system| Immunity

The mucous membranes lining the digestive,respiratory,and urogenital systems have a combined surface area of about 400 metre per square(nearly the size of a basket ball court)and are the major sites of entry for most pathogens.These vulnerable membrane surfaces are defended by a group of organized lymphoid tissues mentioned earlier and known collectively as mucosa-associated lymphoid tissue (MALT). Structurally,these range from loose,barely organized clusters of lymphoid cells in the lamina propria of intestinal villi to well-organized structures such as the familiar tonsils and appendix,as well as Peyer's patches,which are found within the submucosal layer of the intestinal lining.The functional importance of MALT in the body's defense is attested to by its large population of antibody-producing plasma cells,whose number far exceeds that of plasma cells in the spleen,lymph nodes,and bone marrow combined.

The tonsils are found in three locations:lingual at the base of the tongue;palatine at the sides of the back of the mouth;and pharyngeal(adenoids) in the the roof of the nasopharynx.All three tonsil groups are nodular structures consisting of a meshwork of reticular cells and fibers interspersed with lymphocytes,macrophages,granulocytes and mast cells.The B cells are organized into follicles and germinal centers; the latter are surrounded by regions showing T-cell activity.The tonsils defend against antigens entering through the nasal and oral epithelial routes.
 The best studied of the mucous membranes is the one that lines the gastrointestinal tract.This tissue,like that respiratory and urogenital tracts,has the capacity to endocytose antigen from the lumen.Immune reactions are initiated against pathogens and antibody can be generated and exported to the lumen to combat the invading organisms.The lymphoid cells are found in various regions within this tissue.The outer mucoal epithelial layer contains so-called intraepithelial lymphocytes(IELs).Many of these lymphocytes ate T cells that express unusual receptors,which exhibit limited diversity for antigen.Although this population of T cells is well situated to encounter antigens that encounter through the intestinal mucous epithelium.their actual function remains largely unknown.The lamina propria,which lies under the epithelial layer,contains large numbers of B cells,plasma cells,activated T-Helper cells,and macrophages in loose clusters.Histologic sections have revealed more than 15000 lymphoid follicles within the intestinal lamina propria of a healthy child.The submucosal layer beneath the lamina propria contains Peyer's patches,nodules of 30-40 lymphoid follicles in other sites,those that compose Peyer's patches can develop into secondary follicles with germinal centers.
The epithelial cells of mucous membranes play an important role in promoting the immune response by delivering small samples of foriegn antigen from the lumina of the respiratory,digestive,and urogenital tracts to the underlaying mucosal-associated lymphoid tissue.This antigen transport is carried out by specialized M cells.

Structure of M cells :

Structure of M cells and production of IgA at inductive sites.

         The structure of M cells is striking: These are flattened epithelial cells lacking the microvilli that characterize the rest of the mucous epithelium.In addition,M cells have deep invagination,or pocket,in the basolateral plasma membrane;this pocket is filled with cluster of B cells,T cells,and macrophages.Luminal antigens are endocytosed into vesicles that are transported from the luminal membrane to the underlying pocket membrane .The vesicle then fuse with the pocket membrane,delivering the potentially response-activating antigens to the clusters of lymphocytes contained within the pocket.
         M cells are located in so-called inductive sites-small regions of a mucous membrane that lie over organized lymphoid follicles.Antigens transported across the mucous membrane by M cells can activate B cells within these lymphoid follicles.The activated B cells differentiate into plasma cells,which leave the follicles and secrete the IgA into the lumen,where they can interact with antigens.            
        Mucous membranes are an effective barrier to the entrance of most pathogens,which thereby contributes to nonspecific immunity.One reason for this is that the mucosal epithelial cells are cemented to one another by tight junctions that make it difficult for pathogens,including both bacteria and viruses,have exploited the M cell as an entry route through the mucous-membrane barrier.In some cases,the pathogen is internalized by the M cell and transported into the pocket.In other cases,thus allowing entry of the pathogen.Among the pathogens that use M cells in these ways are several invasive Salmonella species,Vibrio cholerae,and the polio virus.