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Immune System

Objective :

  • To learn the procedure for collection of the organs of immune system (spleen, thymus and lymph node)
  • To learn the anatomical structure of the organs of immune system (spleen, thymus and lymph node)

Theory :

  • The immune system functions to protect animals from invading pathogenic microorganisms and cancer. It is able to generate an enormous variety of cells and molecules capable of specifically recognizing and eliminating an apparently limitless variety of foreign invaders. These cells and molecules act together in a dynamic network whose complexity rivals that of the nervous system.
  • A number of morphologically and functionally diverse organs and tissues have various functions in the development of immune responses. These can be distinguished by function as the primary and secondary lymphoid organs. The thymus and bone marrow are the primary (or central) lymphoid organs, where maturation of lymphocytes takes place.
  • The lymph nodes, spleen, and various mucosal associated lymphoid tissues (MALT) such as gut-associated lymphoid tissue (GALT) are the secondary (or peripheral) lymphoid organs, which trap antigen and provide sites for mature lymphocytes to interact with that antigen. In addition, tertiary lymphoid tissues, which normally contain fewer lymphoid cells than secondary lymphoid organs, can import lymphoid cells during an inflammatory response.
  • The thymus is the site of T-cell development and maturation. The thymus is unique among the lymphoid organs in mammals, because its microenvironment consists of reticular epithelium; in birds, the Bursa of Fabricius also has an epithelial framework.  The function of the thymus is to generate and select a repertoire of T cells that will protect the body from infection.
  • The main immunological function of the spleen is to guard the body's vascular compartment. It does so by generating T-cell-independent IgM-antibody responses to bacterial polysaccharides, and by exerting an enormous phagocytic power. Loss of this function occurs following splenectomy. The spleen consists of two main compartments, the red pulp and the white pulp.
  • The red pulp consists of blood-filled sinusoids and cords of Billroth, containing macrophages, lymphocytes, and plasma cells. Macrophages in the red pulp perform a major function in red blood cell clearance and in phagocytosis, especially of nonopsonized particles.
  • The phagocytic function is especially important in early intravascular bacterial infection, before antibody formation and subsequent opsonization occurs. Splenic macrophages and the hepatic phagocytic system together synthesize the majority of complement components involved in the classical complement cascade.
  • Lymph nodes, connected by lymph vessels, consists cortex and medulla. The cortex is the site of antigen encounter and initiation of immune reactions. The products of an immune response (activated cells, effector lymphocytes, inflammatory-mediators) are generated in the medulla. The secretory epithelial surfaces of the body form a major route of entry for potentially pathogenic substances.
  • These surfaces include the epithelium of the gastrointestinal, upper and lower respiratory, and urogenital tracts. The host response at such locations ranges from physical (epithelial barrier, gastrointestinal motility, and respiratory mucociliary ''escalator'') or chemical (low gastric pH, mucus, lysosomal and digestive enzymes) responses, to antigen-specific immune responses.
  • T-cells disseminated in the epithelium and lamina propria of the gastrointestinal tract form the body's largest single T-cell pool. Together with the T-cells in organized mucosa lymphoid sites, such as Peyer's patches and draining mesenteric lymph nodes, these T-cells play a crucial role in the host's defense, and carry out specialized tasks in the initiation and achievement of responses to antigens present in the intestinal lumen.
  • Lymphoid tissue occurs just underneath the secretory epithelium, as seen in Peyer's patches in the duodenum and jejunum, the appendix of the large intestine, and as lymphoid aggregates along bronchi and in the oro- and nasopharyngeal region. These mucosal lymphoid tissues share structural and functional characteristics, and are strongly interrelated.
  • Therefore, the common designation 'mucosa-associated lymphoid tissue' (MALT) is used to cover bronchus-associated (BALT), gut-associated (GALT), and (oro) nasal-associated (NALT) lymphoid tissues.


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