Excerpt from Mucosa-Associated Lymphoid Tissue

Synonyms, Key Words, and Related Terms: lymphoid tissue, lymph node, mucus membrane, mucus, mucosal tissue, tonsils, Peyer patches, Peyer’s patches, vermiform appendix, non-Hodgkin lymphoma, non-Hodgkin’s lymphoma, NHL, lymphoma, malignancy, malignancies, cancer, Hashimoto thyroiditis, Hashimoto’s thyroiditis, Crohn disease, Crohn’s disease, celiac disease, Sjögren syndrome, Helicobacter pylori, H pylori, MALT, gut-associated lymphoid tissue, GALT, bronchial/tracheal-associated lymphoid tissue, BALT, nose-associated lymphoid tissue, NALT, vulvovaginal-associated lymphoid tissue, VALT, MALToma, MALT lymphoma, gastric MALT lymphoma, nongastric MALT lymphoma, gastric MALToma, nongastric MALToma, human mucosa

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Background: A significant quantity of lymphoid tissue is associated with human mucosa. Mucosa-associated lymphoid tissue (MALT) is scattered along mucosal linings, measuring roughly 400 m². These surfaces protect the body from an enormous quantity and variety of antigens. The tonsils, Peyer patches within the small intestine, and the vermiform appendix are examples of MALT. The nomenclature incorporates location; therefore, MALT includes gut-associated lymphoid tissue (GALT), bronchial/tracheal-associated lymphoid tissue (BALT), nose-associated lymphoid tissue (NALT), and vulvovaginal-associated lymphoid tissue (VALT). Additional MALT exists within the accessory organs of the digestive tract, predominantly the parotid gland.

MALT may consist of a collection of lymphoid cells or may include small solitary lymph nodes. Lymph nodes contain a light-staining region (germinal center) and a peripheral dark-staining region. The germinal center is key to the generation of a normal immune response. The location of MALT is key to its function. Stimulation of B lymphocytes leads to the production of immunoglobulin A (IgA) and immunoglobulin M (IgM) within the Peyer patches, preventing adherence of bacteria and viruses to the epithelium, thus blocking entry to the subepithelial layers of the intestine.

Mucosal epithelial surfaces contain M cells, which are specialized cells that are so named because they exhibit microfolds on their luminal surface and because of their membranous appearance. The role of the M cells is absorption, transport, processing, and presentation of antigens to subepithelial lymphoid cells. These subepithelial cells include CD4⁺ type 1 T-helper cells (THCs) and immunoglobulin D/immunoglobulin M⁺ B lymphocytes, the latter of which are antigen-presenting cells (APCs) and function as memory cells interacting with type 1 THCs.

Under these M cells and in close proximity, B lymphocytes, CD4⁺ T lymphocytes, and APCs are found, of which dendritic follicular cells (DFCs) are one type. This group of cells together constitutes a "pocket" of M cells. Within this pocket, an area of follicles associated with the epithelium (follicle-associated epithelium) is observed. These follicles, having true germinal centers, are similar to the follicles of the spleen and lymph nodes.

The direct secretion of secretory IgA onto mucosal epithelia represents the major effector mechanism of MALT. Major accumulations of lymphoid tissue are found in the lamina propria of the intestine. M cells in the intestinal epithelium overlying Peyer patches allow transport of antigens to the lymphoid tissue beneath it.

Another APC is the DFC, which activates some clones of type 1 THCs, although less potently than B lymphocytes. Stimulation of CD28 on type 1 THCs by B7 co-stimulatory molecules results in the secretion of interleukin 2 and gamma-interferon by type 1 THCs. Regulation of the immune response involves the suppression of type 2 THCs (involved in humoral immunity) by gamma-interferon and production of interleukin 10 by type 2 THCs, which inhibits type 1 THCs. Tolerance to antigens results from the lack of a T-lymphocyte response. Often, this is due to failed involvement of B-lymphocyte co-stimulatory molecules or cytokines. Signaling requires more than just receptor stimulation.

The activity of the germinal centers in the follicle-associated epithelium is key to the immune response. The germinal center provides an area where a large number of cells important in the immune response congregate. Early on in the T-cell–dependent immune response, B lymphocytes known as founder cells concentrate in the germinal center, forming the dark zone, where rapid division of these cells occurs.

Selection of B lymphocytes for participation in the immune response occurs based on their interaction with antigen-antibody complexes on the surface of DFCs. This i .....