The body's defense mechanisms

I.  Targets of the body's defense mechanisms

A.  Pathogens -- disease causing organisms

• bacteria
• viruses
• protozoa
• fungi
• parasitic worms

1.  bacteria

• prokaryotes
  • cell wall
  • reproduce asexually -- on average every 20 minutes
• disease producing agent:  toxins
• antibiotics kill bacteria
  • directly -- cause lysis
  • slow growth
    • prevent synthesis of cell wall
    • prevent synthesis of bacterial proteins

2.  viruses

• strand of DNA or RNA surrounded by protein coat
• life cycle:
  • bind to cell via receptor
  • virus enters cell/injects genetic material
  • uses host cell machinery to:
    • replicate its DNA
    • direct synthesis of protein coat
    • thus new viruses quickly form
  • may insert genetic material into chromosome of host cell
    • inactive or dormant
• disease:
  • destruction of host cell
  • insert genetic material next to host cell proto-oncogene -- cancer

3.  Protozoa

• single-celled organisms
• disease:
  • toxin production
  • release of enzymes that prevent cells of host from working properly
• examples:  
  • malaria
  • giardisis

4.  Fungi

• single cells, simple multicellular organisms
• examples:
  • histoplasmosis -- lung infection
  • athlete's foot

5. Parasitic worms

• multicellular organisms
• examples:
  • flutes
  • tapeworms
  • pinworms
• disease:
  • release toxins
  • competition

B.  Cancerous cells

• genetic changes
• cannot regulate cell cycle

II.  Body's defenses -- non-specific mechanisms

• provide barriers that keep substances from entering body
• limit spread of pathogens that break barrier

1.  surface barriers

• skin
  • epidermis with keratin
  • chemical protection of sebum
• mucous membranes
  • line inner body cavities
  • physical defenses:
    • mucous
  • chemical defenses:
    • high acidity -- vagina, urethra
    • anti-bacterial secretions -- lysozyme

2.  phagocytic cells

• scavenger cells that specialize to engulf and destroy particulate matter
• front-line defense
  • neutrophils
  • eosinophils
  • monocytes -- macrophages

3.  natural killer cells (NK cells)

• cells that roam body in search of "suspicious characters"
• cancerous cells usually target

4.  defensive proteins

• proteins in plasma that are anti-microbial
  • interferons
  • complement

a.  interferons

• proteins released by cells infected by viruses before they die
  • attract macrophages and NK cells
  • will protect cells not yet attracted by virus
  • causes these cells to produce proteins that inhibit viral replication
  • thus protects cells from all strains of viruses
  • effective against some cancers (certain types of leukemia) and infections (hepatitis C)

b.  complement

• group of about twenty proteins usually found in blood
• activated to become anti-microbial in two ways:
  • antibody binding
  • proteins in coats of bacteria/viruses/infected cells
• series of reactions occur that punch holes in membrane of pathogen
  • clusters form -- macrophages attracted

5.  Inflammatory reaction

• a nonspecific response of body to injury or invasion by foreign organisms
• four cardinal signs:
  • redness
  • swelling
  • heat
  • pain
• progression outline:
  • basophils/mast cells at site of injury release chemicals
    • attract neutrophils, macrophages
    • increase permeability of capillaries -- swelling
    • cause blood vessels to widen -- heat, redness
  • neutrophils arrive
    • remove microbes, die
    • pus formed
    • macrophages clean up mess

6.  fever

• elevated body temperature
• caused by pyrogens
  • from bugs
  • from macrophages
• pyrogens
  • raise body temperature set point
    • shivering to warm up
  • set point floats back down
    • sweating to dissipate heat
• how does fever help?
  • increases neutrophil activity
  • interferons work better
  • microbes don't reproduce as well
  • immune system more efficient

III.  Body's defenses -- the immune system

A.  General characteristics

• acts on specific targets
  • zero on pathogens and eliminate them
• immunological memory
• systemic response
  • not limited to initial infection site
• recognition of self from non-self

B.  Terminology

• antigen
  • molecule of part of molecule usually found on the surface of pathogens
  • recognized by lymphocytes as non-self
• lymphocytes -- cells of specific defense
  • B-cells -- mature (immunocompetent) in bone marrow
    • make antibodies
    • antigen presentation
  • T-cells -- mature (immunocompetent) in thymus
    • cytotoxic T-cells
    • helper T-cells
  • found in lymph nodes, spleen, secondary lymphoid organs
• macrophages -- antigen presentation
• MHC complex
  • markers of self
  • windows to the cell
• concept of antigen presentation

C.  The cell mediated immune response

1.  T-cells immunocompetent in thymus

• helper T-cells
• cytotoxic T-cells

2.  Infection -- pathogen in lymphatic system/blood

• virus example -- specific body cells infected
  • viral antigens incorporated into MHC complex
  • virgin cytotoxic T-cell binds self/nonself complex
    • cytotoxic T-primed
• macrophage engulfs viruses
  • viral antigens presented on macrophage MHC complex
  • virgin helper T-cell binds macrophage MHC complex
    • macrophage releases IL-1
      • IL-1activates helper T
      • divides into effector helper T and memory helper T
    • effector helper T secretes IL-2
      • IL-2 activates primed cytotoxic T
        • primed cytotoxic T becomes effector cytotoxic T
          • divides into more effector cytotoxic T
            • destroys infected cells -- perforins
          • divides into cytotoxic memory T

D.  Antibody-mediated response

• infection -- virus example
• virgin B-cell (lymph node, spleen) binds antigen
  • virgin B-cell primed
  • processes bound antigen, presents it as part of MHC complex
• primed B activated only if
  • stimulated by IL-2 release
  • binds helper T-cell that recognizes specific self/nonself complex
• activated B cell divides
  • effector B cells -- plasma cells
    • make antibodies to that antigen
  • memory cells

E.  Immunological memory

-  the body's capacity to make a quicker and more intense immune response to any subsequent encounter with the same type of antigen that provoked the primary response

- memory cells formed during a primary response do not engage in battle

• quickly activated into effector cells and more memory cells upon susequent exposure to same antigen

F.  Immunoglobulins

- mechanism of antibody action

• neutralization
• agglutination
• precipitation
• activate complement

G.  Types of acquired immunity

• naturally acquired
  • active
  • passive
• artificially acquired
  • active
  • passive