**3. Mucosa-associated lymphoid tissue (MALT)**

The immune system can recognize a wide range of unknown antigens and elicit an appropriate respond due to the lymphocytes that have a wide variety of antigen receptors [16]. This system has evolved into a system of secondary lymphoid organs such as the spleen, lymph nodes, Peyer's patches, and other MALT, in line with the defense targets [17]. Highly organized secondary lymphoid organs contain architectural domains that facilitate sequential cellular interactions between antigen-presenting cells and lymphocytes and efficiently promote the activation, selection, and differentiation of B and T cells [16]. Therefore, the immunological response becomes more effective.

#### *Bronchus-Associated Lymphoid Tissue (BALT) Histology and Its Role in Various Pathologies DOI: http://dx.doi.org/10.5772/intechopen.99366*

MALT can function independently of the systemic immune system and therefore encompasses the mucosal immune system, which is a crucial part of immunopathology [18]. It plays an important role in immunological defense by eliciting immune responses against specific antigens encountered along the surfaces of all mucosal tissues [19]. Although MALT is anatomically divided into regions, these regions are functionally interconnected under the name of the common mucosal immune system. In this way, events such as antigen presentation and B-cell activation in a mucosal region can trigger the secretion of immunoglobulin A (IgA) in the mucosal regions of different organs [18, 20]. Due to MALT, which mainly functions to produce and secrete IgA along the mucosal surfaces in antigen-specific, T helper 2-dependent reactions, T helper 1 and cytotoxic T-cell-mediated reactions can occur. This may then result in immunotolerance [20, 21].

The best-known representatives of MALT, which contains approximately half of the lymphocytes of the immune system, [22] are gut-associated lymphoid tissue (GALT), nasal-associated lymphoid tissue (NALT), and BALT. However, structures such as conjunctival-associated lymphoid tissue (CALT), larynx-associated lymphoid tissue, and duct-associated lymphoid tissue (DALT) are other MALT representatives [20, 21].

MALT is divided into the two following functional parts: inducer sites and effector sites. Inducer sites include secondary lymphoid tissues, where the clonal expansion of B cells and IgA class transition occur in response to antigen-specific T-cell activation [19]. GALT, BALT, NALT, and CALT in mice, dogs [23], and baboons [24] and DALT in cynomolgus macaques [25] constitute these inducing sites. These sites are known as secondary immune tissues where antigen sampling occurs, and immune responses are initiated. Although there are many differences between inducing sites in various organs, they all contain the same functional segments as follows: lymphoid follicles, interfollicular zones, subepithelial dome zones, and follicle-associated epithelium or lymphoepithelium containing microfold (M) cells [19].

Effector sites distributed as diffuse lymphoid tissue throughout the lamina propria layer on all mucosal surfaces [26] are known as the transport sites of IgA along the mucosal epithelium. After activation and IgA class transition, T- and B cells migrate from inducing sites to these sites [19]. CD4+ and CD8+ T cells, IgA-, IgG- and IgM-plasma cells, B cells, antigen-presenting dendritic cells, and macrophages [19] constitute the cellular content of these effector regions where secreted IgA (S-IgA) is secreted along the mucosal epithelium [27]. Mast cells and eosinophils can occasionally be seen in the interfollicular area. Thus, all the cell types required to initiate an immune response are present here.
