Chap 3

• Adaptive immunity


  Also called specific immunity or acquired immunity
• Definition of adaptive immunity


  • 3rd line of defense after innate immunity
  • Develops in response to infection
  • Also active against cancer cells
  • Made-up of variety of cells (lymphocytes, macrophages) and organs (thymus, lymph nodes)
  • The response is slower (takes few days)
• Levels of defense against infection


  • Non-specific internal defenses (phagocytosis, natural killer cells, inflammation, fever)
  • Barriers (skin, mucous membranes)
  • Specific immune response (cell-mediated immunity, humoral immunity)
• Characteristics of adaptive immunity


  • Self-nonself recognition
  • Antigen specificity
  • Immunological memory
  • Diversity
• Self-nonself recognition


  • The ability to distinguish self from non-self
  • Respond only to non-self molecules
• Antigen specificity


  • Ability to distinguish subtle differences among antigens
  • Respond only to those required, rather than making a random response
  • Antibodies can distinguish between 2 protein molecules that differ in only a single amino acid
• Immunological memory


  The ability to recall previous contact with the foreign molecule, respond with more rapid and larger response
• Diversity


  • Capable to generate diversity in its recognition molecule, allowing it to recognize billion of unique structures on foreign antigen
  • Can recognize single type of organism
• Components of acquired immunity


  • Cellular components (lymphocytes, phagocytes, natural killer cells, dendritic cells)
  • Lymphoid organs (primary - bone marrow, thymus; secondary - lymph nodes, spleen, mucosal associated lymphoid tissues)
• Types of adaptive immunity


  • Humoral immunity (involve B lymphocytes & antibodies)
  • Cell-mediated immunity (involve T lymphocytes & cytotoxin)
• Humoral immunity


  Mediated by antibodies which are produced by B cells
• Humoral immunity


  Eliminates extracellular microbes and foreign molecules (e.g. toxin)
• Cell-mediated immunity


  Mediated by T cells
• Types of T cells

  • Helper T cells
  • Cytotoxic T cells
• Cell-mediated immunity


  Eliminates microorganisms (e.g. intracellular virus and phagocytosed microbes)
• Humoral immunity


  1. B lymphocytes

  2. Bind to microbe

  3. Eliminate microbe
• Cell-mediated immunity


  1. Antigen-presenting cell

  2. Bind to T-cell-receptor

  3. Proliferation and activation of effector cells (cytotoxic T cells, natural killer cells, macrophages)
• Activation of macrophage


  Leading to microbial killing
• Development of lymphocytes


  • Immature T cells in thymus
  • Mature T cells in secondary lymphatic tissues
• Development of B lymphocytes


  • B lymphocytes produced and mature in bone marrow
  • Plasma cells derived from B cells
  • Plasma cells secrete antibody
  • Memory cells have antibody on membrane
• Development of T lymphocytes


  • T cells mature in thymus
  • Naïve T cells with CD4+ mature into helper T cells
  • Naïve T cells with CD8+ mature into cytotoxic T cells
  • T suppressor cells regulate immune response
• Cell-mediated immunity


  Directed against intracellular pathogens, some cancer cells, and tissue transplants
• Antibody-mediated immunity


  Directed against extracellular pathogens
• Humoral immunity


  Mediated by antibodies (immunoglobulins (Ig))
• Humoral immunity


  • Recognizes and eliminates extracellular microorganisms, their toxins & other products
  • Cannot enter cells - only active against extracellular pathogens
• Antibody


  A glycoprotein which mediates humoral immunity
• Antibody structure


  • 2 identical heavy chains (H)
  • 2 identical light chains (L)
  • Held together by disulphide (S-S) bonds
  • Has variable and constant regions
• Variable regions


  Fold together to form antigen-binding site
• Constant regions


  Interact with immune molecules and cells to mediate humoral immunity
• Types of antibody


  • IgG
  • IgA
  • IgD
  • IgE
  • IgM
• IgA


  • Major Ig in external secretions
  • Monomer (in plasma) or dimer (in mucous membrane)
  • Very little in serum
  • Protects surface tissues from pathogens
• IgE


  • Lowest concentration of all Ig
  • Plays role in Type-1 hypersensitivity reactions
  • Responsible for immunity to parasites
• IgD


  • Found in very low concentration in serum
  • Involved in activation of B cells by antigen
• IgG


  • Major Ig in human serum (~80%)
  • Capable of crossing placenta
  • Effective complement activator and opsonization
• IgM


  • Consist 5-10% of total serum Ig
  • First antibody produced in primary response
  • Effective in agglutination and cytolytic reactions
• Primary immune response


  IgM is the first antibody detectable, titers rise early and decline within weeks/months
• Secondary immune response


  Rapid, intense production of IgG, IgM titer rises slower
• Differences between primary and secondary immune response


  • Lag phase is longer in primary response
  • More IgM and less IgG in primary response
  • Fewer antibodies produced in primary response
  • Primary response is weaker than secondary
• Activities in humoral immune response


  1. Recognition phase

  2. Activation phase

  3. Effector phase
• Recognition phase


  1. Naïve B cells exit bone marrow and travel to secondary lymphoid organs

  2. Antigen receptors on B cells bind to antigens

  3. Antigen receptors not cross-linked require helper T cells

  4. Antigen receptors cross-linked do not require helper T cells