MUCOSAL-ASSOCIATED LYMPHOID TISSUE

Introduction

The mucosa-associated lymphoid tissue (MALT) initiates immune responses to specific antigens encountered along all mucosal surfaces. MALT inductive sites are secondary immune tissues where antigen sampling occurs and immune responses are initiated. Effector sites, present as diffuse lymphoid tissue along all mucosal surfaces are the sites of IgA transport across the mucosal epithelium. Though there are many differences between inductive sites in various organs, they all contain the same basic compartments-follicles, interfollicular regions, subepithelial dome regions, and follicle-associated epithelium. The morphologic differences between MALT and other secondary lymphoid tissues, between the MALT sites of differing anatomic locations, and species differences among laboratory animals are described. The morphologic changes in MALT associated with aging, route of nutrition, and genetic mutation (i.e., the nude and SCID mutations) are also discussed. MALT tissues comprise the mucosal immune system which can function independently of the systemic immune system and are, therefore, an important and often overlooked aspect of immunopathology. [1]

The vulnerable membrane surfaces are defended by a group of organized lymphoid tissues mentioned earlier and known collectively as mucosal-associated lymphoid tissue (MALT). Structurally,these tissues range from loose, barely organized clusters of lymphoid cells in the lamina propria of intestinal villi to well-organized structures such as the familiar tonsils and appendix, as well as Peyer’s patches, which are found within the submucosal layer of the intestinal lining. The functional

importance of MALT in the body’s defense is attested to by its large population of antibody-producing plasma cells, whose number far exceeds that of plasma cells in the spleen, lymph nodes, and bone marrow combined.

The tonsils are found in three locations: lingual at the base of the tongue; palatine at the sides of the back of the mouth; and pharyngeal (adenoids) in the roof of the nasopharynx. All three tonsil groups are nodular structures consisting of a meshwork of reticular cells and fibers interspersed with lymphocytes, macrophages, granulocytes, and mast cells. The B cells are organized into follicles

and germinal centers; the latter are surrounded by regions showing T-cell activity. The tonsils defend against antigens entering through the nasal and oral epithelial routes.

Source: https://imgur.com

The best studied of the mucous membranes is the one that lines the gastrointestinal tract. This tissue, like that of the respiratory and urogenital tracts, has the capacity to endocytose antigen from the lumen. Immune reactions are initiated against pathogens and antibody can be generated and exported to the lumen to combat the invading organisms. Lymphoid cells are found in various regions within this tissue. The outer mucosal epithelial layer contains so-called intraepithelial lymphocytes

(IELs). Many of these lymphocytes are T cells that express unusual receptors (_T-cell receptors, or _ TCRs), which exhibit limited diversity for antigen. Although this population of T cells is well situated to encounter antigens that enter through the intestinal mucous epithelium, their actual function remains largely unknown The lamina propria, which lies under the epithelial layer, contains large numbers

of B cells, plasma cells, activated TH cells, and macrophages in loose clusters.

Source: www.apsu.edu, Google images

The epithelial cells of mucous membranes play an important role in promoting the immune response by delivering small samples of foreign antigen from the lumina of the respiratory, digestive, and urogenital tracts to the underlying mucosal-associated lymphoid tissue. This antigen transport is carried out by specialized M cells. The structure of the M cell is striking: these are flattened epithelial cells lacking the microvilli that characterize the rest of the mucous epithelium. In addition, M cells have a deep invagination, or pocket, in the basolateral plasma membrane; this pocket is filled with a cluster of B cells, T cells, and macrophages. Luminal antigens are endocytosed into vesicles that are transported from the luminal membrane to the underlying pocket membrane. The vesicles then fuse with the pocket membrane, delivering the potentially response-activating antigens to the clusters of lymphocytes contained within the pocket.

Source: Kuby, 5^th edition, Unit 2: Cells and organs of immune system, pg 51, Immunology.

Source: Kuby, 5^th edition, Unit 2: Cells and organs of immune system, pg 51, Immunology.

M cells are located in so-called inductive sites—small regions of a mucous membrane that lie over organized lymphoid follicles. Antigens transported across

the mucous membrane by M cells can activate B cells within these lymphoid follicles. The activated B cells differentiate into plasma cells, which leave the follicles and secrete the IgA class of antibodies. These antibodies then are transported across the epithelial cells and released as secretory IgA into

the lumen, where they can interact with antigens.

Reference:

[1] Cesta MF, Normal structure, function, and histology of mucosa-associated lymphoid tissue, Abstarct, PubMed.

[2] Kuby, 5^th edition, Unit 2: Cells and organs of immune system, pg 50-52, Immunology.