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10 Cards in this Set
- Front
- Back
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Describe the structure of the thymus
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-connective tissue capsule with trabeculae that divide the thymus into lobes
-consists of an outer cortex packed with immature T cells and an inner medulla into which cells pass as they mature -both the cortex and medulla are laced with a network of epithelial cells, dendritic cells, and macrophages, which interact physically with the developing thymocytes. -medulla contains Hassel's corpuscles |
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Identify where T cells are produced and the types of T cells produced
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-produced in the bone marrow
-can either have an α and β chain or they can have a γ and δ chain -γδ T cells (these are mainly present in young animals [especially ruminants] and tend to disappear later in life) -αβ T cells are further subdivided into CD4 T cells which provide obligate aid (help) to B cells while CD8 T cells are destined to kill other cells infected with virus |
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Discuss how TCR repertoires are generated
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-two genes RAG-1 and RAG-2 (recombinase enzymes) are induced and they are involved in DNA rearrangement of 2 genetic loci Vα and Vβ
-a β chain is made first from joining a V, D and J segments to a constant domain -the α chain is made next from joining a V and a J segment to a constant domain -this is done by excision of intervening DNA sequences and further excision in the mRNA to join the VDJ segment to a constant region to eventually produce a single V-D-J-C RNA to code for the polypeptide chain |
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Discuss why an extensive T cell repertoire needs to be generated
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In order to recognize a wide variety of antigens
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MHC restriction is dependent on what?
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Dependent on the thymus not the bone marrow
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Describe the process of POSITIVE SELECTION
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-T cells are tested for their ability to bind via their TCR receptors to MHC proteins expressed on cortical epithelial cells of the thymic cortex
-those that can bind with low affinity receive positive selection and go on to the medulla -T cells that do not recognize an MHC molecule are killed by apoptosis |
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Describe the process of NEGATIVE SELECTION
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-the double positive thymocytes migrate to the coricomedullary junction
-negative selection of thymocytes works by inducing apoptotic death of thymocyes whose TCR receptors recognize peptide-MHC complexes in the thymus on thymic DCs with high avitidity -thus negative selection selects against recognizing self -if the T cells recognize MHC I they will be CD4+ T cell and if they recognize MHC II they will be CD8+ T cells |
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Do γδ cells undergo thymic education?
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No, they appear to be exported with a limited TCR repertoire
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What are T regulatory cells?
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-they are cells which originate in the thymus but suppress immune responses in the periphery to self peptides
-these areCD4+ cells with a functional TCR but it is believed that they have a high affinity for MHC self peptides and these thymocytes instead of being deleted by negative selection appear to differentiate into T regulatory cells |
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How many T cells leave the thymus?
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1%
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