Immune system

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Immune system

• Haixu Tang
• School of Informatics

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Human lymphoid organs
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Lymphocytes are required for adaptive immune
responses to foreign antigens
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Innate immune system helps activate the adaptive
immune system
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The development and activation of T and B cells
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The clonal selection theory
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Primary and secondary antibody responses
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Cellular basis of immunological memory
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Induction of immunological tolerance
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B cell activation
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Innate and adaptive immune responses
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An antibody molecule
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Antibody-antigen interactions
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The hinge region of an antibody molecule
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A pentameric IgM molecule
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Antibody-activated phagocytosis
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There Are Five Classes of Heavy Chains

• IgM, which has µ heavy chains, is always the
  first class of antibody made by a developing B
  cell
• After leaving the bone marrow, the B cell starts
  to produce cell-surface IgD molecules as well,
  with the same antigen-binding site as the IgM
  molecules.
• The major class of immunoglobulin in the blood is
  IgG, which is a four-chain monomer produced in
  large quantities during secondary immune
  responses
• IgA is the principal class of antibody in
  secretions, including saliva, tears, milk, and
  respiratory and intestinal secretions
• The tail region of IgE molecules, which are
  four-chain monomers, binds with unusually high
  affinity (Ka 1010 liters/mole) to yet another
  class of Fc receptors

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Antigen binding to antibody
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Constant and variable regions
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The gene of an antibody heavy chain
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3D structure of antibody
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Antigen-binding sites of antibodies
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DNA is rearranged during B cell development
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The V-J joining process
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The human heavy-chain gene-segment pool
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The four main mechanisms of antibody
diversification
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Antibody gene-pool selection in B cell development
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The two main classes of adaptive immune responses
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DNA rearrangement that occurs in class switch
recombination
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T cell responses differ from B cell responses in
two crucial ways

• T cells are activated by foreign antigen to
  proliferate and differentiate into effector cells
  only when the antigen is displayed on the surface
  of antigen-presenting cells in peripheral
  lymphoid organs. Whereas B cells recognize intact
  antigen, T cells recognize fragments of protein
  antigens that have been partly degraded inside
  the antigen-presenting cell. The peptide
  fragments are then carried to the surface of the
  presenting cell on special molecules called MHC
  proteins
• The second difference is that, once activated,
  effector T cells act only at short range, either
  within a secondary lymphoid organ or after they
  have migrated into a site of infection. They
  interact directly with another cell in the body,
  which they either kill or signal in some way.
  Activated B cells, by contrast, secrete
  antibodies that can act far away.

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Two main classes of T cells

• Effector cytotoxic T cells directly kill cells
  that are infected with a virus or some other
  intracellular pathogen.
• Effector helper T cells, by contrast, help
  stimulate the responses of other cellsmainly
  macrophages, B cells, and cytotoxic T cells

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T cell receptor heterodimer
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Activation of a T cell
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Cytotoxic T cells kill their target cells
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Differentiation of naïve helper T cells into
either TH1 or TH2 effector helper cells
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Recognition by T cells of foreign peptides bound
to MHC proteins
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Class I and class II MHC proteins
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Human MHC genes
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A peptide bound in the groove of a class I MHC
protein
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A peptide bound in the groove of a class II MHC
protein
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The interaction of a T cell receptor with a viral
peptide bound to a class I MHC protein
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CD4 and CD8 co-receptors on the surface of T cells
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An effector cytotoxic T cell recognizes some
aspect of the surface of the host target cell
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The processing of a viral protein for
presentation to cytotoxic T cells
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The processing of an extracellular protein
antigen for presentation to a helper T cell
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Positive and negative selection in the thymus
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The two signals that activate a helper T cell
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The signaling events initiated by the binding of
peptide-MHC complexes to T cell receptors
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The stimulation of T cells by IL-2 in culture
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The activation of TH1 and TH2 cells
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Signaling events activated by the binding of
antigen to B cell receptors (signal I)
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The influence of B cell co-receptors on the
effectiveness of signal I
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Comparison of the signals required to activate a
helper T cell and a B cell