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Immune System. Means of Defense (3 categories). First two are Nonspecific A. Barriers Doesn’t distinguish between agents Helps Prevent Entry into the body Includes skin and mucous membranes which provide a mechanical and chemical barrier

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Means of Defense (3 categories)

  • First two are Nonspecific

  • A. Barriers

    • Doesn’t distinguish between agents

    • Helps Prevent Entry into the body

    • Includes skin and mucous membranes which provide a mechanical and chemical barrier

    • Oil and sweat glands give skin an acidic pH which discourages bacterial growth

    • Perspiration, tears, and saliva contain an enzyme lysozyme which breaks down the cell walls of bacteria.

    • Stomach acid kills bacteria

    • Nostril hairs-filter out particles and mucous traps microorganisms. (swept out by cilia)


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Means of Defense (3 categories)

  • B. Nonspecific Internal Defenses

    • 1. Phagocytic cells like neutrophils and monocytes attack microorganisms that get through the skin barriers. They engulf them and consume bacteria, viruses and cell debris in the interstitial fluid.

    • Neutrophils make up 60-70% of WBC’s. Short lived.

    • Monocytes become macrophages which live a long time.

    • Many wander but some are permanent residents in organs like the lung or brain

    • Can engulf 100+ bacteria

    • 2. Natural Killer Cells

    • Destroy the body’s own infected cells-especially those with viruses in them

    • Destroys cells which may be changing and could form tumors

    • Not phagocytic but lytic-breaks cell membranes.


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Means of Defense (3 categories)

  • 3. The Inflammatory Response

  • Triggered by damage to tissues by injury or microorganisms

  • Injured cells give off a substance that aids the inflammatory response

  • release histamine-which causes dilation of neighboring blood vessels and makes the capillaries more leaky (histamine is contained in WBC’s called basophils and in mast cells (in connective tissue)

  • Small blood vessels dilate and this increased blood flow causes the redness, heat and swelling associated with infection. Blood vessels also become leakier causing swelling due to increased fluid.

  • The increased blood flow enhances migration of phagocytic WBC’s from the blood to the interstitial fluid

  • neutrophils first

  • followed by monocytes that develop into macrophages


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More Inflammatory Response

  • Pus- is mostly dead cells and accumulated fluid (will be absorbed)

  • Clotting proteins in plasma seal the area off and prevent the spread of infection.

  • Systemic Reactions

    • the number of WBC’s circulating is increased ( may be dramatic and within hours)

      • fever

      • toxins from the pathogens may trigger fever.

      • Some WBC’s release pyrogens which set the body thermostat higher

      • The higher temperature stimulates phagocytosis and inhibits the growth of microorganisms and may speed up repair.


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Still More Non-specific Response

  • 4. Antimicrobial Proteins- Some proteins attack microorganisms or negatively affect their reproduction.

    • A. Interferon- a substance produced by a virus infected cell that helps other cells resist the virus

    • -The virus turns on the interferon gene- can save the infected cell but diffuses to nearby cells and inhibits viral reproduction there

    • -Host specific-not virus specific

    • -Inteferon may act against cancer since some may be induced by viruses

    • one kind mobilizes natural killer cells-destroys tumor cells

    • may change malignant cell membranes- make them less likely to metastasize

    • activates macrophages


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Another Antimicrobial Protein

  • B. Complement

    • many (at least 20) proteins that work with other defense mechanisms

    • They circulate in the blood in an inactive form

    • They are activated by the immune response or markers on microorganisms

    • They increase the inflammatory response- histamine release and attraction of phagocytes.

    • They coat microbes- this increases the rate of phagocytosis by opsonization (“making tasty”)

    • They team up to cause lysis of the microbial membrane


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The Third Line of Defense

  • C. Specific Immune Response

    • The immune system learnsto distinguish “self” from “non-self”

    • Detects “antigens”- foreign substances . These cause an increase in the cells that attack or produce antibodies. They may be things like molecules on surfaces of viruses and bacteria or marker molecules on transplanted organs.

    • The immune response must be activated by the presence of an antigen

    • The response is specific for a particular foreign substance or invader

    • The system remembers the antigens after the initial contact. There is a quick response the second time. This is memory.

    • Vaccination- immune response is prevented with a nonvirulent or weakened form of a pathogen-initiates long term capability to respond quickly to real infective agent.

    • -Active immunity- body produces antibodies

    • -Passive immunity- body acquires antibodies from other sources such as placenta, injection, immunity is temporary