**1. Introduction**

Mucosal surfaces interact directly with the outside of the body and interact with countless antigens. The need to establish an immune system in this tissue to fight pathogens is obvious, but the development of an immune response against native antigens or bacterial bacteria is an undesirable response. Therefore, the immune system in the mucosal tissues must be tolerant of many antigens, while maintaining the ability to respond to a small number of pathogenic antigens. Any tissue that can secrete mucus on the surface of the epithelial layer and can participate in the immune response is considered part of the mucosal lymphatic tissue (MALT). MALT is present in the gastrointestinal tract, airways, urogenital tract, conjunctiva, and endocrine glands (salivary and sweat glands), but has been studied mainly in the gastrointestinal tract, respiratory tract, and urogenital tract. Both innate and adaptive immune systems (humoral and cellular) are seen in these tissues. One of

the defense mechanisms in the mucosa is the physical and mechanical defense that acts as a non-specific barrier against infections, including the mucosal epithelial layer, intestinal peristaltic activity, and the mechanism of mucosal-mucosal clearance in the airways. The first line of defense in the mucosa is physical defense and innate immunity. Innate immune cells, such as tissue-resident macrophages and migrating neutrophils, are the first cells to act upon the onset of pathogen exposure. After innate immunity, adaptive immunity and its cells are activated by dendritic cells in the marginal lymph nodes (or in organized mucosal-associated lymphoid tissue) and called to the sites of infection. B cells in mucosal tissues produce and secrete antibodies, especially IgA. T lymphocytes also play a role in secreting proinflammatory cytokines or inducing cytotoxic activity. Moreover, mucosal tissues contain populations of Tαβ and Tγδ [1, 2].

#### **1.1 Lymphatic tissues in the gastrointestinal tract**

The human gastrointestinal tract consists of a tubular structure covered by a mucosal epithelial layer. Beneath the epithelial cells is the lamina propria, or lining of the mucosa, which contains the mucosal connective tissue (MALT), blood vessels, and lymph vessels. MALT located in the gastrointestinal tract is also called GALT1 . MALT in this area also contains a large number of immune cells, which alone are larger than any other set of bone marrow, thymus, spleen, and lymph node cells. Mucosal lymph tissue is mainly composed of intraepithelial lymphocytes (IELs), lamina propria lymphocytes, IgA-producing plasma cells and macrophage antigen-presenting cells, dendritic cells, neutrophils, eosinophils, and mast cells. In certain areas of the mucosa, there are lymphoid follicles that contain T lymphocytes, B lymphocytes, etc. In general, it can be said that the intestine prevents the entry of bacteria and infectious agents in three ways, the first is through the mucosal layer that prevents the penetration of bacteria from the epithelium. The second barrier is the production and secretion of antimicrobial peptides in the intestinal lumen and killing them within the lumen. The third method of inhibition is the production of IgA from the plasma of lamina propria, which neutralizes pathogens within the intestinal lumen [3, 4].

#### **2. The role and structure of mucosal lymph tissues**

Mucous lymphatic tissues can be classified according to their structure and function. Structurally, mucosal lymph nodes are divided into two categories: organized or O-MALT<sup>2</sup> and diffuse or D-MALT3 . Functionally, O-MALT is known as the site of induction of the immune response and D-MALT is the site of the immune response. In other words, immune responses are formed in O-MALT and perform their executive function in D-MALT. O-MALT is a place for antigen processing and production of effector and memory cells, after which the produced cells migrate to other mucosal diffuse lymph tissues such as D-MALT, leading to the protection of body surfaces. However, it has recently been shown that both types of lymph tissue play an important role in the production and differentiation of mucosal lymphocytes and mucosal immunity. Epithelial cells also play a role in the differentiation and production of cytotoxic T cells. It seems that intestinal mucosa and other mucosal surfaces affect bone marrow progenitor cells (T and B cells) and are effective in

<sup>1</sup> Gut Associated Lymphoid Tissue.

<sup>2</sup> Organized Mucosal Associated Lymphoid Tissue.

<sup>3</sup> Diffuse Mucosal Associated Lymphoid Tissue.

#### *Mucosal Immunology DOI: http://dx.doi.org/10.5772/intechopen.98863*

gene rearrangement of immunoglobulins and T cell receptors. The activation of the enzymatic machine required for the genetic synthesis of progenitor cells in the gut supports this theory. T cells also regulate the activity of epithelial cells. For example, intercellular permeability and ion secretion (by these cells) are affected by IFN-γ. Crypt cell proliferation in the small intestine and mucosal morphology are also regulated by T cell cytokines. O-MALT is called the afferent lymphoid region, which is the site of antigen entry and the formation of immune responses. While D MALT is an efferent lymphoid region and acts as a site of antigen interaction with differentiated cells (leading to antibody secretion and the activity of helper and cytotoxic lymphocytes) [1, 4, 5].
