Gut-associated lymphoid tissue
|Gut-associated lymphoid tissue|
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.
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.
The following comprise lymphoid tissue in the gut:
- Waldeyer's tonsillar ring
- Peyer's patches
- Lymphoid aggregates in the appendix and large intestine
- Lymphoid tissue accumulating with age in the stomach
- Small lymphoid aggregates in the esophagus
- Diffusely distributed lymphoid cells and plasma cells in the lamina propria of the gut
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 cells 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.
The digestive tract is an important component of the body's immune system. In fact, the intestine possesses the largest mass of lymphoid tissue in the human body. The GALT is made up of several types of lymphoid tissue that store immune cells, such as T and B lymphocytes, that carry out attacks and defend against pathogens.
The adaptive immunity, mediated by antibodies and T cells, is only found in vertebrates. Whereas all of them have a gut-associated lymphoid tissue and the vast majority have a version of spleen and thymus, not all vertebrates show bone marrow, lymph nodes or germinal centers, what means that not all vertebrates can generate lymphocytes in bone marrow. This different distribution of the adaptive organs in the different groups of vertebrates suggests GALT as the very first part of the adaptive immune system in vertebrates. It has been suggested that from this existing GALT, and due to the pressure put by commensal bacteria in gut that coevolved with vertebrates, later specializations as thymus, spleen or lymph nodes appeared as part of the adaptive immune system.
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