Gastrointestinal immunology

image of Gastrointestinal immunology
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This chapter is made up of three parts: 5a Gastrointestinal immunology; 5b Biopsy: sample collection; 5c Biopsy: handling, processing and interpretation. The gastrointestinal (GI) tract has the highest concentration of immunological tissue within the body, which is not surprising given the continual bombardment of the GI mucosal surface with antigenic material. A clear knowledge of the anatomy and function of the GI system is essential for understanding the pathogenesis and management options for the spectrum of immune-mediated diseases that affect the GI tract (e.g. inflammatory bowel disease; dietary hypersensitivity; small intestinal bacterial overgrowth; antibiotic-responsive diarrhoea).

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5.1 Section of a canine tonsil. Squamous epithelium (arrowed) covers the organized lymphoid tissue (LT) of the organ. The salivary gland (SG) is present deep to the lymphoid area.
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5.2 Section of canine gastric mucosa showing an unencapsulated lymphoid aggregate.
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5.3 Section of a Peyer’s patch from a feline ileum. Note the extension of lymphoid tissue up into modified areas of ‘dome’-shaped mucosa.
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5.4 (a) Section of canine villus showing the enterocytes and underlying lamina propria, which contains a mixture of mononuclear cell types. (b) Section of canine villus immunohistochemically labelled to stain T lymphocytes (expressing the CD3 molecule) golden brown. Intraepithelial and lamina propria T cells are evident.
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5.5 Diagram showing the major compartments of the small intestinal GALT; the epithelium, lamina propria and Peyer’s patch. Lymphoid cells activated by exposure to antigen in the Peyer’s patch or lamina propria migrate in afferent lymph to mesenteric lymph nodes. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. IEL = intraepithelial lymphocyte; LPL = lamina propria lymphocyte.
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5.6 Section of canine colonic mucosa. There are scattered mononuclear cells (arrowed) within the normal lamina propria.
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5.7 The basic immune response. Foreign antigen first contacts the innate immune system and, after processing by a dendritic cell, there is subsequent activation of the T and B lymphocytes of the adaptive immune system. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. NK = natural killer cell; Tc = T cytotoxic cell; Th = T helper cell; Treg = T regulatory cell.
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5.8 Response of the GALT to a pathogenic organism. Pathogen-derived antigen most likely enters the Peyer’s patch via the M cell. Antigen-specific T and B lymphocytes are activated within the patch or the mesenteric lymph node and must circulate through the bloodstream to enter the intestinal lamina propria to mount the ‘effector phase’ of the immune response. Some activated lymphocytes enter other mucosal tissues as part of the ‘common mucosal’ immune system. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. DC = dendritic cell; PC = plasma cell.
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5.9 Secretion of IgA across the epithelial barrier. IgA dimers (two IgA monomers linked by the J chain) are secreted by a lamina propria plasma cell. The dimer binds to the polymeric Ig receptor (pIgR) on the basal surface of an enterocyte and this complex is internalized by the cell. The complex passes through the cell cytoplasm to become re-expressed on the luminal surface. The dimer is released, carrying a small portion of the pIgR as the ‘secretory piece’ that protects it from proteolysis within the intestinal lumen. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.10 Mechanisms of oral tolerance. Antigens from food or endogenous microflora enter the lamina propria and activate populations of regulatory T lymphocytes that prevent a full-scale immune response being mounted. These regulatory cells may also act systemically to prevent reactions to the same antigens encountered parenterally. PC = plasma cell; T = T regulatory cell. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.11 Section of liver labelled immunohistochemically to show expression of the class II molecules of the major histocompatibility complex by Kupffer cells and dendritic cells (stained brown) within sinusoids and the space of Disse.
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5.12 Hepatic immune system. A range of antigen-presenting cells (APCs) and lymphocytes migrate from the sinusoidal blood to the space of Disse. Other resident immune cells are found within the portal areas of the liver. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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