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Anne ([email protected]) Tresha ([email protected]) Lymphoid System I: Peripheral System, Lymph Node To put this in context: Your acquired immune system is made of B and T cells, each specific for a single antigen. B and T cells are born in the bone marrow and then matured in the primary lymph organs (bone marrow and thymus respectively). They then circulate in the blood as naïve lymphocytes. Having never “seen” their antigen, they leave the blood through high endothelial venules (HEV) to filter through secondary lymph organs – lymph nodes, Peyer’s patches, and the spleen – in hopes of finding their perfect match. Antigen is concentrated in these 2o lymph organs by several mechanisms including afferent lymphatic vessels, M cells, and antigen presenting cells (APC’s). APC’s act as sentinels, scouting for antigen in the skin and mucosa. When they find antigen, they eat it and quickly travel with the lymph via lymphatic vessels to the nearest lymph node. In the node, lucky and specific B cell and T cells interact with the antigen in follicles, proliferating and maturing. Mature B and T cells then exit via the blood vessels to fight off the infection. Mature B cells may also stay in the lymph node as plasma cells, secreting their major protein product, antibody. Lymph Vessels: -More permeable then blood vessels -Drain extracellular space, delivering free antigen and cells (APC’s) to lymph nodes, connecting nodes. -Valves prevent backflow -Thinner, less obvious than veins Lymph Follicle: Site of antigen encounter and proliferation/differentiation of B and T cells. Primary Follicle: Secondary follicle -No germinal center -Antigen has been encountered -Small lymphocytes, naïve -Germinal center = site of B cell proliferation! Stains pale b/c of many plasmablasts with large, euchromatic nuclei and prominent nucleoli. Some plasmablasts will be in mitosis. -T cells are helping at the periphery B cell 1 2 Lymphoid Diffuse Lymphoid Tissue: GALT, S/B/MALT, tonsils, appendix, etc. -Found in the lamina propria of GI tract, respiratory tract, GU tract, where pathogens are likely to invade. -No capsule GALT: Peyer’s patches in gut -Antigen enters via transcytosis across special M cells in the epithelium. NO afferent lymphatics. Tonsil: ring of diffuse lymphatics in oropharynx -Crypts of stratified squamous non-keratinized epithelium -Lymphocytes can be seen invading epithelium Anne ([email protected]) Tresha ([email protected]) Encapsulated Lympoid Tissue: Spleen, Lymph nodes: Lymph flow w/i node: Afferent lymphatics subcapular sinus trabecular sinuses medullary sinuses efferent lymphatics Lymph Node: -Capsule: Dense Irregular CT Trabeculae are extensions of the capsule -Subcapsular, trabecular space Macrophages (visualized with India ink) -Cortex: Outermost area = follicles (B cell) Deep cortex = T-zone - post-capillary venules (“high” endothelial venules = cuboidal or columnar) Tcells enter & exit blood stream here! -Medulla Made of cords and sinuses Plasma cells (mature and darker staining B cells) secrete antibodies -Hilum: indented area surrounded by the medulla CT, fat, blood vessels (afferent and efferent), nerves, efferent lymphatics with valves. Bielschowsky silver stain *Capsule is DICT (collagen I stains red-brown) *Stroma is reticular tissue (collagen III stains brownblack) Anne ([email protected]) Tresha ([email protected]) Q1) For the two pictures on the slide, select the correct answer for each of the below: i) ii) iii) iv) Is encapsulated Contains germinal centers Peripheral lymphoid tissue Central lymphoid tissue Q2) The region at the arrow is: A) B) C) D) Peyer’s Patch Germinal Center Lymph node Primary lymphoid nodule Q3) The region inside the box contains which of the following upon antigenic stimulation: A. B. C. D. E. Plasma Cells Macrophages T-Cells Memory B-Cells All of the Above Answers Q1) A = tonsil Lab 11, Slide 2 B = lymph node Lab 11, Slide 9 i) B ii) C (Both) iii) C (Both) iv) D (Neither) The thymus and bone marrow are the central lymphoid tissues Q2) D Lab 11, Slide 4 This lymphatic tissue in the gut is formed into a lymphatic nodule, but there is no germinal center so it is primary. It is not a Peyer’s patch because that is a group of lymph nodules in the GI tract. Q3) E. Plasma cells are secreting antibody into efferent lymph. Macrophages continue to filter the medullary sinuses with the efferent lymph. Memory T and B-cells exit the node via the efferent lymph node.