**3.3 GI-tract tissues anatomy**

If we want to look at the mechanism of action of the digestive system under a magnifying glass, it is necessary to know which organs and which tissues we should examine.

The GI-tract, which begins in the esophagus and ends in the anus (i.e., esophagus, stomach, liver, gallbladder, pancreas, small-intestine, appendix, large intestine, rectum, and anus), has different tissues, biochemical and biophysical functions, and mechanisms [70]. Each of which must be examined separately and their functions considered together. After swallowing, food enters the stomach through the esophagus (passes through the muscular cuff) and by mechanical and chemical activity in the stomach, food enters the duodenum (beginning of the small intestine) entirely as a concentrated liquid containing digestive acids and enzymes (i.e., gastric juice) then, due to physical and chemical activities, the intestine is fully digested to provide the materials and compounds needed by the body. To make it easier, two important GItract organs (i.e., stomach and intestines), tissues, and their physical and chemical functions are summarized below [70–72]:

Gaster


*Large Association of GI Tract Microbial Community with Immune and Nervous Systems DOI: http://dx.doi.org/10.5772/intechopen.104120*

> Intestine **Tissue** ¼ **1** � **Mesentery 2** � **Vesseles** ¼ **Vein Artery Nerve** ¼ **Submucosal plexus Myenteric plexus** 8 < : 8 >>>>>>>>< >>>>>>>>: **3** � **Muscularis** ¼ **Circular muscle Longitudinal muscle** 8 < : **4** � **Serosa** ¼ **Areolar connective tissue Epitheulium** 8 < : **5** � **Submucosa** ¼ **Lymphatic tissue Glands Lumen** 8 >>>>< >>>>: **6** � **Mucosa** ¼ **Epithelium Lamina propia Muscularis mucosae** 8 >>>>< >>>>: 8 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>< >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>: **Physical function** ¼ **Small intestine Ileum**, **Duodenu**, **jejunum** � � ¼ *Peristalsis Segmentation Absorption* 8 >>>>< >>>>: **Large intestine** ð Þ *ascending*, *descending and sigmoid colon* <sup>¼</sup> *Peristalsis Segmentation Absorption* 8 >>>>< >>>>: 8 >>>>>>>>>>>>>>>>< >>>>>>>>>>>>>>>>: **Chemichal function** ¼ **Small intestine** ¼ **Lipid Breackdown Pancreatic lipases Phospholipase** 8 < : **protein Breakdown Trypsin Chymotrypsin Carbopeptidase Elastase** 8 >>>>>>>>>< >>>>>>>>>: **Strach Breakdown Pancreatic amylase Dextrinase Sucrose Maltase Lactase Amyloglucosidase** 8 >>>>>>>>>>>>>>>>>>>< >>>>>>>>>>>>>>>>>>>: **Nucleic Acid Breakdown Nucleases Nucleosidases Phosphatases** 8 >>>>>< >>>>>: 8 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>< >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>: **Large intestine** ¼ f**Fermentation** ¼ **fermentation products** 8 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>< >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>: 8 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>< >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>:

### **3.4 GI-tract immune system anatomy**

The immune system can be defined as a complex system that protects the body against microorganisms, infectious agents, and a variety of autoimmune diseases and carcinogens, immune system can respond to any antigen in both specific and nonspecific forms. The immune system function can also be seen in both innate and adaptive forms in all systems, infrastructures, and various cellular and molecular mechanisms to stop or eliminate invasive antigens [73].

Apart from the lymphoid cell, various organs and cells are also involved in intestinal immunity (i.e., goblet cells, entero-endocrine cells, macrophages, mast cells), these appropriate subsets of lymphoid cells are usually found in the epithelium (e.g., T suppressor) or in the lamina propria (e.g., T helper), IgA is also mostly produced by plasma cells [74]. The inductive sites are organized into specialized aggregations of lymphoid follicles called Peyer's patches, are demonstrated as typical organized lymphatic structures of the intestine. They are present and found in large numbers from before birth to the senescence and also present in the ileum, duodenum, and jejunum [74, 75]. In superior vertebrates, such as mammalians, the immune system is made up of primary and secondary lymphatic organs that are organized in an almost identical morphology. The thymus and bone marrow are the major organs of the primary lymphatic system, and the spleen, lymph nodes, and mucosal- associate lymphoid tissue (MALT) are the secondary lymphatic system. Innate immunity is found in all living things and can detect protected and common molecular structures in pathogens and microorganisms. These include the identification of polysaccharides, lipopolysaccharides (LPS), peptidoglycans, bacterial and viral DNA and RNA through the interaction of specific receptors (e.g., toll-like receptor TLR) [73].

The GI-tract has the largest volume of microbes in the human body, maintaining an elegant balance between immunity against pathogens and tolerance toward commensal microbiome, such as immune balance, or intestinal homeostasis, is accomplished by fine-tuning and cooperating with various branches of the immune system, including the innate and adaptive immune system [76]. The gastrointestinal mucosa separates the digestive fluid inside the duct, which contains a large number of antigens from various sources, and prevents the antigens from freely reaching the body, it also allows for some vital host and peripheral intestinal interactions. The mucosal immunity is related to secretory IgA; The IgA is derived from mucosal plasma cells after the proliferation of its precursors in antigen-induced Peyer patches. In fact, IgA is transported to the intestinal tract after binding to the secretory component (SC) as a dimer. However, the induction of local immunity and intestinal systemic tolerance may be a specific immune response to the gut-associated lymphoid tissue (GALT) [77].

Therefore, the immune system can deal with any pathogen in different conditions, depending on the location, amount and type of damage, and all this is due to the chemical structures at the cellular and molecular levels of organisms. Chemical structures help identify the invasive agent and the type of response to them. These structures, which are generally proteins, are produced and secreted by epithelial, endothelial, dendritic cells (DCs), and lymph nodes and are commonly known as cytokines [73].

In total, the number of proteins that have cytokine activity reaches more than 200, their secretion depends on the effective concentrations of cytokines that are created in the vicinity of target cells [78]. Cytokines are involved in the interaction of lymph

### *Large Association of GI Tract Microbial Community with Immune and Nervous Systems DOI: http://dx.doi.org/10.5772/intechopen.104120*

cells, hematopoietic cells, and inflammatory cells. They are usually having a short half-life but the network of cytokine activity is such that it communicates between all cells and factors involved in the immune system. Also, the inflammatory responses, regulation of hematopoiesis, proliferation control, and cellular differentiation are different biological responses that can induce by cytokines [11, 79].

Cytokines are a general name for a complex of proteins that are involved in our immunity in the form of structural molecules. This complex including of lymphokine (cytokines produced by lymphocytes), chemokines (cytokines with chemical activity), interleukins (cytokines produced by leukocytes that affect other leukocytes), and monokine (cytokines produced by monocytes). All of these cytokines can work together and can even counteract the effects of each other. Also, cytokines stimulate B- and T-cell-dependent responses. In the immune system, T-cells respond well to the activation of B-cells in response to antigens, the proliferation and the activation of eosinophils, neutrophils, and basophils by cytokines. The cytokines act by binding to specific receptors on the target cell membrane. So far, four types of receptor proteins for cytokines have been identified that are classified into five families including immunoglobulin receptors, class I cytokine receptors (hematopoietins), class II cytokine receptors (interferons), TNF receptors, and chemokine family receptors [11, 12].

As mentioned earlier, the gut contains the largest immune system and intestinal mucus is considered as the primary site of interaction with common and pathogenic organisms. The innate immune system acts to restrict the passage of microbiota through the mucosal barrier, so intestinal epithelial cells, in coordination with antigen-presenting cells (APCs), form the first line of defense in the intestine. Cytokine binding to the T-cell receptor promotes T-cell expansion or expression of distinct Th subsets or to regulatory T cells (Tregs). Th1 cells produce proinflammatory cytokines, including IFN-γ and TNF-α, which are important for cell-mediated immunity against most bacteria. In contrast, Th2 cells produce anti-inflammatory cytokines, including IL-4 and IL-13, which are critical for humoral mediated immunity against extracellular pathogens. Cytokines bind to cell surface receptors in immune and nonimmune cells, activating the JAK–STAT signaling pathway and positively regulating intestinal function by regulating the expression of specific target genes [80].
