Chapter 15

• Host Defense Mechanisms are ways in which the body protects itself from pathogens - referred to as 3 lines of defense
• The first 2 lines of defense are nonspecific, the 3rd line of defense, the immune response, is very specific
• Antibodies


  Special proteins produced in response to foreign substances called antigens
• Lines of Defense

  • First Line
  • Second Line
  • Third Line
• Nonspecific host defense mechanisms


  General mechanisms that serve to protect the body against many harmful substances
• Innate resistance


  Exact factors that produce innate resistance 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 (e.g. pH, temperature, perspiration, enzymes)
  • Microbial antagonism
• Transferrin


  Glycoprotein that increases in response to systemic bacterial infections, binds to iron to deprive 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 IL-1
• Interferons


  Small antiviral proteins produced by virus-infected cells, prevent viruses from multiplying
• Complement System


  • Group of about 30 different proteins in normal blood plasma, interact in a stepwise manner known as the complement cascade, assist in the destruction of many different pathogens
  • Opsonization is a 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, some are chemoattractants that recruit phagocytes
• Inflammation


  1. Increase in capillary diameter (vasodilation)

  2. Increased capillary permeability allowing escape of plasma and proteins

  3. Exit of leukocytes from capillaries and accumulation at site of injury
• Inflammation


  • Localizes infection, prevents spread of microbial invaders, neutralizes toxins, aids tissue repair
  • Causes redness, heat, swelling, and 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


  Thick, greenish-yellow exudate containing many live and dead leukocytes, also known as pus
• Phagocytosis


  Process by which phagocytic white blood cells (phagocytes) surround and engulf (ingest) foreign material
• Types of Leukocytes

  • Monocytes
  • Lymphocytes
  • Granulocytes (eosinophils, basophils, neutrophils)
• Opsonization


  Coating of microbes with complement components, such as C3b, making them more easily phagocytosed due to the presence of complement receptors on phagocytic cells
• Ingestion Phase of Phagocytosis

  1. Attachment

  2. Pseudopodia extend around bacterial cell

  3. Pseudopodia fuse

  4. Bacterial cell is within the phagocyte
• Digestion Phase of Phagocytosis

  1. Lysosome with digestive enzymes moves toward phagosome

  2. Lysosome membrane fuses with phagosome membrane

  3. Lysosome and phagosome become a single membrane-bound vesicle

  4. Bacterial cell is digested within the phagolysosome
• Mechanisms by which pathogens escape destruction by phagocytes include capsules, exoenzymes that kill phagocytes, and not being destroyed within the phagolysosome
• Disorders and conditions that adversely affect phagocytic and inflammatory processes include leukopenia, defects in leukocyte motility and chemotaxis, and chronic granulomatous disease
• Additional factors that can impair host defense mechanisms

  • Nutritional status
  • Increased iron levels
  • Stress
  • Cancer and cancer chemotherapy
  • Genetic defects
  • Age
  • AIDS
  • Drugs (e.g. steroids)
• Leukopenia 


  an abnormally low number of circulating leukocytes