Gut-associated lymphoid tissue (GALT) is a component of the mucosa-associated lymphoid tissue (MALT) which works in the immune system to protect the body from invasion in the gut. Owing to its physiological function in food absorption, the mucosal surface is thin and acts as a permeable barrier to the interior of the body. Equally, its fragility and permeability creates vulnerability to infection and, in fact, the vast majority of the infectious agents invading the human body use this route. The functional importance of GALT in body's defense relies on its large population of plasma cells, which are antibody producers, whose number exceeds the number of plasma cells in spleen, lymph nodes and bone marrow combined. GALT makes up about 70% of the immune system by weight; compromised GALT may significantly affect the strength of the immune system as a whole.
Structure The gut-associated lymphoid tissue lies throughout the intestine, covering an area of approximately 260–300 m2. In order to increase the surface area for absorption, the intestinal mucosa is made up of finger-like projections (villi), covered by a monolayer of epithelial cells, which separates the GALT from the lumen intestine and its contents. These epithelial cells are covered by a layer of glycocalyx on their luminal surface so as to protect cells from the acid pH. New epithelial cells derived from stem cells are constantly produced on the bottom of the intestinal glands, regenerating the epithelium (epithelial cell turnover time is less than one week). Although in these crypts conventional enterocytes are the dominant type of cells, Paneth cells can also be found. These are located at the bottom of the crypts and release a number of antibacterial substances, among them lysozyme, and are thought to be involved in the control of infections. Underneath them, there is an underlying layer of loose connective tissue called lamina propria. There is also lymphatic circulation through the tissue connected to the mesenteric lymph nodes. Both GALT and mesenteric lymph nodes are sites where the immune response is started due to the presence of immune cells through the epithelial cells and the lamina propria. The GALT also includes the Peyer's patches of the small intestine, isolated lymphoid follicles present throughout the intestine, and the appendix in humans. The following examples comprise lymphoid tissues that act as interfaces between immune system and incoming antigens either as food antigens or as pathogenic or commensal microbiota's antigens:
Waldeyer's tonsillar ring Small lymphoid aggregates in the esophagus Lymphoid tissue accumulating with age in the stomach Peyer's patches in the small intestine Diffusely distributed lymphoid cells and plasma cells in the lamina propria of the gut Intraepithelial lymphocytes (IELs) interspersed into epithelial layer of mucosal surfaces Lymphoid aggregates in the appendix and large intestine Mesenteric lymph nodes draining lymph coming from the gut tissue GALT can be also divided into two categories considering the structure, from which the function arise. There can be found 1.) organised GALT made up from folicules – such as Peyer's patches, mesenteric lymph nodes and even more organised appendix. Its main function is to induce immune reaction. 2.) diffuse GALT with single T and B cells, macrophages, eosinophiles, basophiles and mast cells, preferentially found in lamina propria. This part of GALT is made up from mature effector cells ready to perform their actions. The GALT has been described in the adult eastern grey kangaroo (Macropus giganteus), tammar wallaby (Notamacropus eugenii), stripe-faced dunnart (Sminthopsis macroura), and red-tailed phascogale (Phascogale calura). The adult northern brown bandicoot (Isoodon macrourus) has been described to have both organised and diffuse GALT. The development of the GALT has also been described in several marsupial species, including tammar wallabies, stripe-faced dunnarts (Sminthopsis macroura), and red-tailed phascogales
Peyer's patches
The Peyer's patch is an aggregate of lymphoid cells projected to the lumen of the gut which acts as a very important site for the initiation of the immune response. It forms a subepithelial dome where large number of B cell follicles with its germinal centers, T cell areas between them in a smaller number and dendritic cells are found. In this area, the subepithelial dome is separated from the intestinal lumen by a layer of follicle-associated epithelium. This contains conventional intestinal epithelial cells and a small number of specialized epithelial cells called microfold cells (M cells) in between. Unlike enterocytes, these M cells present a folded luminal surface instead of the microvilli, do not secrete digestive enzymes or mucus and lack a thick surface of glycocalix, so it can be in contact with microbiota and antigens presented in the content of gut.
Function
Under normal circumstances, immune system of the whole organism needs intestinal source of antigens to train and regulate development of various immune cells. Without having such stimulation, many properties of immune systems do not develop, as it is shown on the case of germ-free animals. Because immune cells are in constant touch with bacterial and food antigens, the primary response is set up as tolerogenic. The complex interaction between these intestinal microbiota, the intestinal epithelial layer, and the local mucosal immune system is essential for maintaining gut health and systemic immunity because the nutrition level of a person's diet contributes to the gut microbiota (a multispecies microbial community of bacteria, fungi, and viruses, all in a particular niche) that is in synergy with the host. Still there must be a robust defence in a case that pathogens cross either the border line of epithelium or produce harmful substances like bacterial toxins. Such a compromise is ensured by diverse types of immune cells:
… excerpt ends here. Continue reading the full article.
