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Animal Defenses

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					Animal Defenses


Specific and Nonspecific Defenses
Animal Defenses
   Nonspecific defense
    – First line of defense
       • skin
       • mucous membranes
       • secretions of skin and mucous membranes
    – Second line of defense
       • phagocytes (usually WBCs)
       • antimicrobial proteins
       • inflammatory response
   Specific Defense
    – Lymphocytes
    – Antibodies
Animal Defenses

   Nonspecific, first line defenses
    – Skin is a physical barrier
    – Mucous membranes are physical barriers
    – Both also produce secretions that can be
      antimicrobial
       • sebaceous fluid and sweat
       • saliva, tears, mucous secretions
Animal Defenses
   Nonspecific, second line defenses
    1. Phagocytic cells
      •   Neutrophils - WBCs that migrate to sites of
          infection by chemotaxis and engulf invaders;
          kamikaze cells
      •   Monocytes - WBCs that leave the blood
          stream and become macrophages; kill with
          superoxide anion, nitric oxide, or lysozymes
      •   Eosinophils - WBCs that attack parasites by
          releasing destructive enzymes (much less
          phagocytic activity than the others)
      •   Natural killer (NK) cells - WBCs that attack
          cells infected by viruses or cancer cells; not
          phagocytic but form complexes that punch
          holes in cells causing them to explode
Animal Defenses
   Nonspecific, second line defense
    2. The inflammatory response
      –   Produced by tissue damage due to injury or
          invasion of microorganisms
      –   Precapillary arterioles in the damaged area
          dilate while postcapillary venules constrict
      –   This causes redness and heat
      –   Capillaries become leakier and fluid leaks into
          the injured tissue causing swelling
          – Basophils (WBC) and mast cells release histamine
            which causes both dilation and increased
            permeability
          – Prostaglandins (released by basophils and other
            WBCs and by damaged tissue) also contribute
    Animal Defenses
    Nonspecific, second line defense
    2. The inflammatory response
       –   Increase blood flow and capillary permeability allows for an
           enhanced transport of phagocytic cells to the area
       –   Chemotactic factors and chemokines attract them to sites of
           injury
       –   Neutrophils arrive first, then monocytes that become
           macrophages
       –   Macrophages not only engulf pathogens, but also dead
           neutrophils, and damaged tissue
       –   Pus is an accumulation of dead phagocytes, and proteins
           and fluids that have leaked out of the capillaries
       –   If the infection causes more than minor damage, chemical
           signals are sent out that cause WBC production to speed
           up
       –   Usually, more neutrophils are made first
Animal Defenses
   Nonspecific, second line defense
    3. Fever
      – Toxins produced by pathogens can cause an
        elevation of body temperature
      – Some WBCs release pyrogens, which can
        also cause an increase in body temperature
      – Pyrogens actually reset the body’s thermostat
        to a higher temperature
      – This is a favorable situation, up to a point
      – A moderate fever will
         – inhibit microbial growth
         – speed up chemical reactions so that phagocytosis
           and tissue repair can occur more quickly
Animal Defenses
   Nonspecific, second line defense
    – The problem with #2 and #3
    – Septic shock occurs when the infection causes a
      systemic inflammatory response and high fever
    – If all capillaries are perfused and become leaky,
      so much fluid will move out of the blood that blood
      pressure will drop drastically
    – If there is little to no pressure difference between
      the left and right sides of the heart, what
      happens?
    – Also, the high fever that accompanies septic
      shock can cook you
    – So, a little is good, a lot is not
Animal Defenses
   Nonspecific, second line defense
    4. Antimicrobial proteins
      –   Lysozymes are present in tears, saliva, and
          mucous secretions
      –   Complement - a set of 20+ proteins found in
          blood and tissues that act like an extracellular
          signaling pathway that leads to microbe
          destruction; these proteins are also part of the
          specific defense system
      –   Interferons - when cells become infected with
          viruses, they secrete substances called
          interferons that effect the behavior of
          neighboring cells, causing them to produce
          substances that will prevent their infection; some
          interferons can also enhance phagocytic activity
          of phagocytes
Specific Animal Defenses
   The body’s third line of defense involves
    lymphocytes
    – B lymphocytes - b for bone marrow
       • These cells have receptors that can bind to
         specific parts of a foreign substance
       • The receptors can be called antigen receptors or
         membrane-bound antibodies
    – T lymphocytes - t for thymus
       • These cells also have receptors that can bind to
         specific parts of a foreign substance
       • The receptors are called T cell receptors
Specific Animal Defenses
   Antigen - any foreign molecule that elicits a specific
    immune response
   Antibody - aka immunoglobulin; a protein secreted
    by and found on the surface of B lymphocytes;
    membrane-bound antibodies elicit responses,
    secreted antibodies are the start of responses;
    secreted antibodies attack pathogens so they can
    be disposed of;
   Clonal selection - each antigen will only find a select
    few lymphocytes who have receptors that can bind
    to it; once selected, these few lymphocytes will
    undergo many rounds of mitosis to produce
    thousands of cells, all of which are specific for and
    dedicated to the elimination of that one particular
    antigen
    Specific Animal Defenses
   Clonal selection produces two kinds of
    cells
    – Effector cells - cells that are actively involved
      in the elimination of antigen
       • For B lymphocytes, effector cells are called
         plasma cells and they secrete large amounts of
         antibodies specific to the antigen
       • For T lymphocytes, effector cells are called
         effector T cells
    – Memory cells - cells that will be kept in
      “storage” in case another infection occurs at
      a later date
    Specific Animal Defenses
    Specific immune responses can be:
    1. Primary immune response
      – occurs upon the first exposure to an antigen
      – takes 10-17 days to be in full swing
      – so at first the antigen causes illness, then a
        recovery is seen
    2. Secondary immune response
      –   occurs some time after the initial exposure
      –   takes only 2-7 days to be in full swing
      –   an illness may never occur
      –   the response is greater and more prolonged
      –   also known as immunological memory and is
          the basis for immunizations
 Specific Animal Defenses
 During the cell
  differentiation process
  of lymphocytes,
  lymphocytes containing
  receptors that can bind
  to molecules in the
  body are selected for
  apoptosis, or to
  become anergic
 This creates a self-
  tolerance, a critical
  aspect of the immune
  system
Specific Animal Defenses
   Major histocompatibility complex (MHC)
    – Genes that encode for cell surface
      glycoproteins
    – These were discovered in the process of
      studying tissue and organ rejection
    – Two kinds of MHC genes are actually
      necessary for the selection of specific T
      cells during their development
    – Class I MHC molecules - select for
      cytotoxic T cells (TC) that can bind to them
    – Class II MHC molecules - select for helper
      T cells (TH)
Specific Animal Defenses
 Class I MHC molecules are found on all of the body’s
  cells
 Class II MHC molecules are found only on antigen-
  presenting cells (APCs)
   – Macrophages and B cells for example
 Macrophages and B cells put pieces of antigen onto
  the class II MHC molecules
 TH cells bearing receptors that can bind to both the
  piece of antigen and the class II MHC, are now
  activated
 Activated TH cells can now activate TC cells and B
  cells to initiate a primary immune response
 Activated TC cells can now destroy any cell that has
  the same antigen presented on a class I MHC
  molecule
               membrane attack
                  complex

Opsonization
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    Invertebrate Defenses
 Even sponges can determine self from non-self
 Disposal of non-self is usually by phagocytosis
 Most invertebrate defense is nonspecific but in
  some species there are immunoglobulin
  molecules
    – In insects, for example, hemolin, found in
      hemolymph, binds to microbes and aids in their
      elimination
    – In earthworms, they exhibit some specificity and
      immunological memory
       • If you graft a piece of body wall tissue from one earthworm
         on to a different earthworm, the tissue will be rejected in
         about 2 weeks
       • If you do a 2nd graft using the same donor, the graft will be
         rejected in just a few days
Specific Immunity: Friend or Foe
    Active immunity - immunity acquired in
    response to an infection
    – If active immunity is acquired due to an
      immunization/vaccination, it is called artificial
      active immunity
   Passive immunity - antibodies are passed
    from an immune animal to a non-immune
    animal
    – Antibodies pass from mother to fetus across the
      placenta, and from mother to infant in colostrum
    – Immunoglobulin shots from physicians
Specific Immunity: Friend or Foe
   Blood transfusions
    – Type A blood has B antibodies and A antigens
    – Type B blood has A antibodies and B antigens
    – Type AB blood has no antibodies and A & B
      antigens
    – Type O blood has A & B antibodies but no
      antigens
   Tissue grafts and organ transplantations
    – Rejection is due to MHC molecules on cell
      surfaces
    – In bone marrow, we see graft versus host
      reactions
   In both cases, typing must be done to be sure
Specific Immunity: Friend or Foe
   Allergies - hypersensitive reactions of the
    immune system to environmental antigens,
    called allergens
    – Mast cells are called into play because of
      activation from antibodies, causing the release of
      histamine and an inflammatory response
    – Anaphylactic shock is when an allergic response
      becomes systemic and blood pressure drops
      drastically
   Autoimmune diseases - when the immune
    cells no longer recognize certain cells as self
    (lupus, insulin-dependent diabetes,
    rheumatoid arthritis, MS)
Specific Immunity: Friend or Foe

   Immunodeficiency diseases - severe
    combined immunodeficiency (SCID) is a
    genetic disease that effects both the T
    and B lymphocytes; AIDS is caused by
    a virus, the human immunodeficiency
    virus (HIV)
    – There are people who are resistant to HIV
      infection due to a defective CCR5 receptor
      on the surface of their TH cells

				
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