# The Gastrointestinal System ![[Pasted image 20240508232618.png|300]] ![[Pasted image 20240508232629.png|300]] ![[Pasted image 20240508235336.png|400]] ## Mucosal Tissue **Mucus** is a slimy, protective secretion composed of: - Glycoproteins - Proteoglycans - Peptides - Enzymes It is produced by *goblet cells* in many internal [[Histology|epithilia]] Examples of mucosal epithelial surfaces: - Linings of the gastrointestinal tract - Eyes - Nose - Throat - mammary glands - Respiratory tract - Genitourinary Tract - Urinary tract - Genital tract **Mesenteric Lymph Nodes** are the largest [[Lymph Nodes|lymph nodes]] found within the body and are located close to mucosal [[Epithelium 1|epithelial]] surfaces **Peyer’s patches** are sites of organized gut-associated lymphoid tissue found only in the small intestine. Covered with the M cell–rich, follicle-associated epithelium, these structured lymphoid organs contain T cell and B cell zones and a dome region. The appendix is a secondary lymphoid organ associated with the first part of the large intestine, attached to the cecum. Throughout the entire intestine, isolated lymphoid follicles serve the areas between the larger lymphoid organs and contain B cells. ### Antibody Function Within Mucus Secreted and membrane-associated mucin molecules and the C-terminal regions of α and μ heavy chains have cysteine residues that form disulfide bonds. This tethers the antibodies to mucus. --- The sites in the body that contain the largest and most diverse populations of microorganisms are the respiratory and gastrointestinal tracts. These mucosal tissues are associated with secondary lymphoid tissue, known as mucosa-associated lymphoid tissue (MALT). The gut-associated lymphoid tissue (GALT) includes the tonsils, adenoids, the appendix, and Peyer’s patches of the gastrointestinal tract.  GALT is different from lymph node or spleen in their appearance but is similar to them in their function of trapping pathogens to activate lymphocytes. Spleen is a secondary lymphoid tissue where adaptive immune responses are produced against blood-borne pathogens. Spleen is not associated with GALT. Thymus is a primary or central lymphoid tissue where T lymphocyte develop and mature and is not associated with GALT. --- IgA does not activate complement or opsonize microbes. IgA limits the exposure of commensal and pathogenic microorganisms to the mucosa and acts to prevent unnecessary damage to those tissues. --- Transcytosis delivers dimeric IgA and IgM across gut epithelium using the poly-Ig receptor, which binds to these antibodies via their J chains. IgG transcytoses from the lamina propria to the mucosal surface through FcRn. IgA helps limit inflammation by avoiding activating inflammatory processes such as fixing complement and inducing phagocytosis, which IgG and IgM both induce. --- Attached to the cecum and filled with lymphoid follicles, this secondary lymphoid organ is around 10cm in length and 0.5cm in diameter. Infection of this organ can lead to appendicitis and subsequent surgical removal. ---