15. Nonspecific Host Defense Mechanisms

• Host defense mechanisms


  Ways in which the body protects itself from pathogens—referred to as three lines of defense

  
• Lines of defense


  • First line
  • Second line
  • Third line
• First two lines of defense

  • Nonspecific
• Third line of defense

  • Immune response, very specific

  
• Antibodies


  Special proteins produced in response to foreign substances called antigens
• Categories of host defense mechanisms

  • Nonspecific/innate
  • Specific/adaptive

  
• Nonspecific host defense mechanisms

  General, serve to protect the body against many harmful substances

  
• Innate resistance


  Exact factors that produce it are not well understood
• Other nonspecific host defense mechanisms

  • Mechanical and physical barriers to invasion
  • Chemical factors
  • Microbial antagonism
  • Fever
  • Inflammatory response
  • Phagocytic white blood cells

  
• First line of defense

  • Skin and mucous membranes as physical barriers
  • Cellular and chemical factors
  • Microbial antagonism
• The nose has internal hairs, which act as a physical barrier to infection. Cells in the nose produce mucus which traps pathogens before they can enter the lungs. When the nose is blown, mucus is removed and any pathogens are trapped within it.
• Human sweat contains an antibiotic called dermcidin which may serve as a first line of defence against microbial pathogens
• Transferrin


  Glycoprotein that increases in response to systemic bacterial infections, binds to iron depriving pathogens of this vital nutrient
• Fever


  Stimulated by pyrogenic substances, augments host's defenses by stimulating leukocytes, reducing available free plasma iron, and inducing the production of interleukin 1
• Interferons


  Small antiviral proteins produced by virus-infected cells, prevent viruses from multiplying
• Types of interferons


  • Alpha
  • Beta
  • Gamma
• Complement system


  Group of about 30 different proteins found in normal blood plasma, "complementary" to the immune system, interact in a stepwise manner known as the complement cascade to assist in the destruction of pathogens
• Opsonization


  Process by which phagocytosis is facilitated by the deposition of opsonins (e.g., antibodies or certain complement fragments) onto objects (e.g., pathogens)
• Acute-phase proteins


  Plasma proteins that increase rapidly in response to infection, inflammation, or tissue injury
• Cytokines


  Chemical mediators released from many different types of cells, enable cells to communicate within the immune system and between the immune system and other systems of the body
• Chemokines


  Cytokines that are chemoattractants, recruit phagocytes to sites where they are needed
• Inflammation


  1. Increase in capillary diameter (vasodilation)

  2. Increased capillary permeability

  3. Exit of leukocytes from capillaries and their accumulation at site of injury
• Purposes of inflammation


  • Localize an infection
  • Prevent the spread of microbial invaders
  • Neutralize any toxins being produced at the site
  • Aid in the repair of damaged tissue
• Signs and symptoms of inflammation

  • Redness
  • Heat
  • Swelling (edema)
  • Pain
• Sequence of events in inflammation

  1. Tissue injury

  2. Vasodilation

  3. Increased permeability

  4. Emigration of leukocytes

  5. Chemotaxis

  6. Phagocytosis
• Inflammatory exudate


  Accumulation of fluid, cells, and cellular debris at the inflammation site
• Purulent exudate/Pus


  Thick, greenish-yellow exudate containing many live and dead leukocytes
• Pyogenic microbes


  Pus-producing microbes such as staphylococci and streptococci
• Phagocytosis


  Process by which phagocytic white blood cells (phagocytes) surround and engulf (ingest) foreign material
• Major categories of leukocytes

  • Monocytes
  • Lymphocytes
  • Granulocytes
• Types of granulocytes


  • Eosinophils
  • Basophils
  • Neutrophils
• Important phagocytes


  • Macrophages
  • Neutrophils
• Four steps in phagocytosis


  • Recognition and attachment
  • Ingestion
  • Digestion
  • Elimination
• Capsules initially serve to protect pathogens from phagocytosis
• Some bacteria produce an exoenzyme called leukocidin, which kills phagocytes
• Some bacteria (e.g., Mycobacterium tuberculosis) are not destroyed within the phagolysosome
• Disorders and conditions that adversely affect phagocytic and inflammatory processes

  • Leukopenia
  • Disorders affecting leukocyte motility and chemotaxis
  • Disorders affecting intracellular killing by phagocytes (e.g., chronic granulomatous disease)
  • Nutritional status
  • Increased iron levels
  • Stress
  • Cancer and cancer chemotherapy
  • Various genetic defects
  • Age
  • AIDS
  • Drugs (e.g., steroids)