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CARDIOVASCULAR - BIOS 255 Anatomy And Physiology III With Lab
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BIOS255 – Anatomy & Physiology IIIProf Haneen Salhieh, M.S.
Week 5: Immune System
Immune System
Immunity is defined as the body’s ability to ward off the pathogens that produce disease. body's ability to
maintain homeostasis is highly dependent on its ability to fight off pathologic agents such as bacteria,
fungi, viruses, or other disease-causing microorganisms. The body must also protect itself against
nonliving disease agents as well (e.g., allergens, latex, poison ivy toxin, etc.).
The body consists of three lines of defense against threats.
The first and second lines of defense are forms of innate immunity, and the third line of defense is a form
of adaptive immunity.
First Line of Defense: Innate immunity (Skin, mucous membranes – resp, reproductive,
digestive, & chemical secretions sweating, stomach acid, tears. saliva)
Second Line of Defense: Innate immunity - Interferons/ complement proteins (Neutrophils,
Eosinophils, Basophils, Monocytes, NK Cells) – lead to inflammation (heat, redness, swelling,
pain)
^When first two lines of defense fail then 3rd line activates
Third Line of Defense: Adaptive immunity
Creates memory by remembering pathogen to attack quickly for next time.
T Cell (cell-mediated which actually destroys the pathogen) - CELLULAR
B Cell (antibody-mediated which tags the pathogen but does not destroy) – HUMORAL
Innate: Nonspecific (1st and 2nd)
Adaptive: Specific (3rd)
, BIOS255 – Anatomy & Physiology III
Prof Haneen Salhieh, M.S.
Innate Immunity: First Line of Defense
Innate immunity consists of defense mechanisms that are present from birth. This is also known as
nonspecific immunity as it protects us against a wide range of pathogens and foreign substances. This
form of immunity encompasses the first and second lines of defense.
The body’s first line of defense includes physical and chemical barriers that safeguard against the
invasion of microorganisms. These structures are the initial and immediate protective mechanisms that
effectively prevent the entry of pathogens. This line of defense is composed of skin, mucous membranes,
and chemical barriers.
Skin
Skin is the largest organ of the body that forms an external barrier to all internal and underlying
structures. The epidermal layer of skin is mainly composed of the tough protein keratin that few
pathogens can penetrate. The top layers are shed frequently to wash off the pathogens that may
have colonized.
A thin film of lactic and fatty acids called the acid mantle inhibits bacterial growth.
Skin has antimicrobial chemicals that defend against pathogens. Sweat contains antibacterial
peptides called dermcidin, along with other peptides called defensins produced by other cells of
the skin. These peptides destroy pathogens and increase resistance to infection.
Mucous Membranes
Digestive, respiratory, urinary, and reproductive tracts are open to the exterior and can act as a
portal of entry for pathogens. These tracts are protected by mucous membranes that produce a
sticky mucus that physically traps microorganisms. Pathogens trapped in respiratory mucus are
moved upward by cilia to either the stomach to be swallowed and destroyed by stomach acid or
excreted out via the oral cavity.
Microorganisms trapped in the upper digestive tract are destroyed by stomach acid and flushed
from the lower urinary tract by urine.
Chemical Barriers
Perspiration (sweat) contains a high-level of salt in sweat inhibits pathogen growth.
Sebum (oil) helps keep skin pliable, reducing the chances of tear and abrasions which can
compromise this barrier.
Stomach acid contains a very low pH that pathogens cannot thrive in, inhibiting their growth and
reproduction.
Mucus, tears, and saliva contain an enzyme called lysozyme that destroys bacteria by dissolving
their cell walls.
Flushing helps remove microbes (saliva, tears, urine)
Innate Immunity: Second Line of Defense
Should the body’s physical and chemical barriers fail and pathogens break through, the second line
of defense comes into action. This line of defense is composed of protective cells that can do
both phagocytic and non-phagocytic killing, antimicrobial proteins, and protective mechanisms such as
fever and inflammation. Let’s start by examining the different types of protective cells.
Neutrophils continually wander in connective tissue and kill bacteria. They are the first to arrive
as a response to injury or infection. These phagocytic cells engulf microorganisms and destroy
them with their lysosomal enzymes. Whenever neutrophils detect nearby pathogens, their
lysosomes discharge enzymes and catalyze a reaction called the respiratory burst. Here, they
form a chemical killing zone that destroys the bacteria but can unfortunately also destroy the
neutrophil itself due to the potent oxidizing agents produced from the reaction.
Eosinophils are leukocytes found in mucus membranes and are active during parasitic worm
infections, allergies, and inflammatory reactions. Parasitic worms too large to be destroyed by
phagocytosis are killed by chemicals released by these granulocytes. Eosinophils also release an
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