Mucosa-Associated Lymphoid Tissue
The mucosal lining of the alimentary canal and airways is in many ways specialised to facilitate the exchange of substances between the external environment and the body. Unfortunately, these specialisation do not just apply e.g. to components of the digested food but also pathogens. This is combined with excellent living conditions for bacteria in parts of the alimentary canal - in particular the ileum and the colon. Lymphoid tissue located beneath the mucosal epithelia, mucosa-associated lymphoid tissue (MALT), protects the body against pathogens that may enter the body via the mucosa. The importance of this task is reflected in the mass of the MALT, which corresponds to the combined mass of the other lymphoid organs and tissues.
The task that the immune cells of the MALT have to accomplish is different from that of other parts of the immune system. We do need a defense against pathogens, but it would not be a good idea to mount an immune response against components of the food. Immune cell activation therefore differs between the MALT and other lymphoid tissues.
This difference is mediated by different receptors expressed by immune cells of the MALT and by different substances which they release upon contact with an antigen. Because of their specific functions, immune cells of the MALT do not mingle with other immune cells. Epithelial cells of the vessels supplying the MALT express specific receptor which are recognized by MALT immune cells and allow their homing to the MALT during recirculation. Lastly, MALT plasma cells produce a secretable form of antibodies, immunoglobulin type A dimers, which can be taken up by epithelial cells and then released onto the epithelial surface.
Specialisation of MALT immune cells occur at the molecular level. In routine histological preparations, immune cells of the MALT look pretty much like immune cells of other lymphoid tissues.
This difference is mediated by different receptors expressed by immune cells of the MALT and by different substances which they release upon contact with an antigen. Because of their specific functions, immune cells of the MALT do not mingle with other immune cells. Epithelial cells of the vessels supplying the MALT express specific receptor which are recognized by MALT immune cells and allow their homing to the MALT during recirculation. Lastly, MALT plasma cells produce a secretable form of antibodies, immunoglobulin type A dimers, which can be taken up by epithelial cells and then released onto the epithelial surface.
Specialisation of MALT immune cells occur at the molecular level. In routine histological preparations, immune cells of the MALT look pretty much like immune cells of other lymphoid tissues.
Often MALT consists of small accumulations of lymphoid cells or one to a few lymph follicles beneath the epithelium and possibly extending into the submucosa. The tonsilsand Peyer's patches are large accumulations of lymphoid tissue with associated specialisations of the epithelium.
0 Comments