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Immunity

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Immunity, Immune System, Immune Status, Immunology, Innate Immunity, Adaptive Immunity

  • See Also
  1. Humoral Immunity (Immunoglobulin)
  2. T-Lymphocyte
  3. B-Lymphocyte
  4. Antigen Processing
  5. Complement Pathway
  6. Immunodeficiency
  • Definitions
  1. Innate Immunity (Natural Immunity)
    1. Generalized, immediate immune response not reliant on prior exposure
  2. Adaptive Immunity
    1. Organism specific Immunity that relies on prior "memory" of exposure
  3. Phagocyte (and Phagosome, Phagocytosis)
    1. Immune cells (Neutrophils and Monocytes/Macrophages) are White Blood Cells that engulf pathogens and foreign material
    2. Phagosomes are the membrane engulfed pathogens
    3. Often combined with lysis by lysozymes
  4. Lysosome (and lysozyme)
    1. Lysozyme-containing vacuoles produced in cellular golgi apparatus, and fuse with Phagosomes, resulting in pathogen lysis
  5. Opsonin (and Opsonization)
    1. Proteins (e.g. Antibody, complement, C-Reactive Protein) that bind a pathogen surface, targeting it for Phagocytosis
  • Types
  • Innate Immunity (Natural Immunity)
  1. Physical Barriers
    1. Skin
    2. Mucosa (e.g. respiratory and gastrointestinal tract)
    3. Cilia (e.g. respiratory)
  2. Secretion Contents
    1. Lysozyme (e.g. in tears, Saliva and in Neutrophils)
      1. Enzymatically degrades cell walls
    2. Acid destroys acid-labile organisms
      1. Sweat Lactic Acid
      2. Stomach Gastric Acid (Hydrochloric Acid)
  3. Phagosomes (Phagocytosis)
    1. Phagocytes such as Neutrophils (PMNs) and Macrophages attract and engulf organisms (Phagocytosis)
    2. Phagocytes attract organisms which in turn activate Phagocytosis
    3. Phagosomes are later lysed via Lysosomes (as below)
      1. Phagocytosis.jpg
  4. Lysosomes
    1. Neutrophil's and Macrophage's (Phagocytes) golgi apparatus produce Lysosomes (vacuoles) that contain lysozyme
    2. Lysosomes fuse with Phagosomes to produce phagolysosomes, degrading the engulfed organisms
    3. Lysosomes may also release their contents extracellularly to lyse larger targets too large to engulf
  5. Natural Killer Cells (NK cells)
    1. Lymphocytes providing protection against Intracellular Bacteria and viruses
    2. NK cells bind Major Histocompatibility Complex 1 (MHC-1)
      1. MHC-1 is present on normal cells and it inactivates NK cells
      2. MHC-1 is NOT expressed by infected cells
        1. NK activating receptor Ligand is expressed
        2. NK cells bind infected cells and destroy them
    3. NK cell mechanisms of infected cell destruction
      1. Cytoplasmic granules
        1. Perforin
          1. Generates pores on cells targeted for destruction
        2. Granzyme
          1. Induces programmed cell death (apoptosis) on entry into Target Cells
      2. Cytokines
        1. Interferon-Gamma (IFN-g)
          1. Activates Macrophages for Phagocytosis
    4. Image: NKC Mediated Destruction of Infected Host Cells
      1. NaturalKillerCells.jpg
  6. Complement Pathway
    1. Images
      1. idComplementPathway.jpg
    2. Activation
      1. Classical Pathway (C1, C2, C3, C4)
        1. C1 binds Antigen-Antibody complex
      2. Alternate Pathway (Properdin, Factor B, Factor D, C3)
        1. Activation via microbe cell surface
      3. Lectin Pathway (Mannose Binding Lectin or MBL)
        1. Mannose Binding Lectin (MBL) binds mannose on microbe surface
        2. Mannose Binding Lectin Associated Proteases (MASP-1, MASP-2) are activated
        3. Classical Pathway (above) is stimulated
    3. Enzyme C3 Convertase (C3bBb or C4b2a) Formation
      1. Enzyme C3 Convertase splits C3 into C3a and C3b
      2. C3a stimulates inflammation (attracts Neutrophils, histamine release)
      3. C3b stimulates Phagocytosis, inflammation (as with C3) and lysis (see below)
    4. Opsonization
      1. Microbe coated with an Opsonin, an Antibody or complement (e.g. C3b)
      2. Surface Opsonins target microbes for Phagocytosis
    5. Phagocytosis
      1. Phagocytes such as Neutrophils (PMNs) and Macrophages attract and engulf targeted organisms
    6. Inflammation (via C3a, C5a)
      1. Chemoattraction of Neutrophils
      2. Anaphylatoxic activation of Mast Cells and Basophils to degranulate, releasing histamines and vasoactives
        1. Inflammation occurs when histamine-induced capillary dilation results in fluid and protein release
    7. Lysis
      1. C3b splits C5 into C5a and C5b
      2. Membrane attack complex or MAC (C5b, C6, C7, C8, C9) binds microbe surface
      3. MAC promotes microbe lysis
  • Types
  • Adaptive Immunity
  1. Humoral Immunity (B-Cells and Antibodies)
    1. Humoral Immunity (i.e. antibodies) targets extracellular pathogens
    2. B Cells
      1. Derivation
        1. Fetal Liver
        2. Bone Marrow Pluripotent Stem Cells
      2. Peripheral Migration to Secondary Lymphoid Tissue
        1. Spleen
        2. Lymph Nodes
        3. Peyer's Patches (Small Bowel)
      3. Activation
        1. Images
          1. BCellActivation.jpg
        2. Recognition
          1. Antigen binds B-Lymphocyte Surface Receptor (BCR)
          2. BCR binding activates B-Lymphocyte
            1. T-Cell Independent Antigen (e.g. inert Antigens) alone activate B-Cells
            2. T-Cell Dependent Antigen (e.g. microbes) require added stmulus (e.g. T Cells)
        3. B-Cell Proliferation
          1. Activated Lymphocytes proliferate
        4. B-Cell Differentiation
          1. Plasma Cells (Antibody producing cells)
            1. Survive for days to weeks producing antibodies, and without replicating
          2. Memory Cells
            1. Remain in B-Lymphocyte pool ready to respond to the same Antigen in future
            2. Future Antigen response is known as secondary immune response
    3. Antibodies
      1. Images
        1. idAntibody.jpg
      2. Immunoglobulin (Ig)
        1. Immunoglobulins (or antibodies) are Y-Shaped glycoproteins generated by Plasma Cells
        2. Immunoglobulin stem (Fc) is composed of 2 identical heavy chains
        3. Two Immunoglobulin Arms emanate from the stem
          1. Each arm is composed of 2 heavy chains and 2 light chains
          2. The end of each arm contains an Antigen binding site (Fab)
        4. Immunoglobulins have 2 forms
          1. Membrane bound Immunoglobulins (on surface of B-Cell)
          2. Secretory Immunoglobulin (unbound, free-floating)
            1. Monomeric antibodies exist as single Antibody molecules (IgE or IgG)
            2. Multimeric antibodies exist as multiple joined antibodies (IgA or IgM)
              1. Connected with J Chains
      3. Immunoglobulin G (IgG and subclasses IgG1-4)
        1. Monomer accounting for 75% of all Antibody, and has a serum half-life of 23 days
        2. Responsible for long lasting Immunity (secondary immune response) and Type 2 Hypersensitivity
      4. Immunoglobulin A (IgA and subclasses IgA1, IgA2)
        1. Dimer (2 Antibody molecules) when secretory Ig and accounts for 10-15% of all Antibody
        2. Serum half life of 6 days
        3. Present in body secretions (e.g. tears, Saliva, milk) and responsible for mucosal Immunity
      5. Immunoglobulin M (IgM)
        1. Pentamer (5 Antibody molecules) when secretory Ig
        2. Responsible for early, primary Antibody response
      6. Immunoglobulin E (IgE)
        1. Long stem (Fc) monomeric Antibody with serum half-life of only 2.5 days
        2. Reacts to allergans (Type 1 Hypersensitivity) and Parasitic Infections
      7. Immunoglobulin D (IgD)
        1. Monomer with serum half life of 3 days
        2. Membrane bound surface Antibody
  2. Cell-Mediated Immunity (T-Cells)
    1. Cellular Immunity (i.e. T Cells) target Intracellular Pathogens (e.g. viruses and Intracellular Bacteria)
    2. T-Cells
      1. Derived in Bone Marrow
      2. Migrate to Thymus
        1. Maturation and Differentiation into two cell lines with different T-Cell Receptors (CD4 and CD8)
      3. Release into peripheral circulation
    3. T-Cell Surface Receptors
      1. T-Cell Receptors (TCR)
        1. Bind the Antigen on the Antigen Presenting Cell
        2. TCR Types
          1. TCR-alpha-beta (TCRab+)
          2. TCR gamma-delta (TCRgd+)
      2. T-Cell Co-Receptors
        1. CD4 binds MHC Class 2 - peptide/Antigen complex on surface of Antigen Presenting Cells (APC)
          1. Only Dendritic Cells, Macrophages, B-Cells (B-Lymphocyte) present MHC Class 2
        2. CD8 binds MHC Class 1 - peptide/Antigen complex on surface of Antigen Presenting Cells (APC)
          1. Any nucleated cell can present MHC Class 1
    4. T-Cell Types
      1. Effector Cells
        1. T-Helper Cells (CD4+ Cells)
          1. Releases Interferon
            1. Stimulates Phagocytosis by Macrophages
            2. Activates Natural Killer Cells
            3. Suppresses viral replication
          2. Releases interleuken 2
            1. Promotes T-Cell proliferation (esp. memory cells)
            2. Promotes B-Cell proliferation (memory cells and plasma cells)
        2. T-Cytotoxic Cells (CD8+ Cells)
          1. Target and destroy tumor cells and virus-infected cells
      2. Other Cells
        1. Memory Cells
        2. Apoptosis of some cells not otherwise differentiated
    5. Naive T-Cell Activation
      1. TCellActivation.jpg
      2. T-Cell Receptor (TCR) binds to MHC-Antigen complex on Antigen Presenting Cells
      3. T-CellSurface CD28 binds to B7 Ligand on Antigen Presenting Cell
      4. T-CellSurface LFA-1 (Lymphocyte Function Associated Antigen) binds ICAM1 on Antigen Presenting Cells
      5. Interleukin-2 (IL2) produced by naive T Cells
        1. Stimulate T Cell proliferation
  • Resources
  1. Immune System (Wikipedia)
    1. https://en.wikipedia.org/wiki/Immune_system
  • References
  1. Mahmoudi (2014) Immunology Made Ridiculously Simple, MedMaster, Miami, FL
  2. Guyton and Hall (2006) Medical Physiology, p. 419-50