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31.1 Infectious disease caused by pathogens
Germ theory
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 Germ theory was pioneered by the work of several scientists whose discoveries each built
upon one another
Louis Pasteur
 Pasteur(a French chemist) is best known for his method of sterilizing milk (pasteurization)
using gentle heat
 Pasteur made several observations involving interference with alcohol fermentation by
microorganisms
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Joseph Lister
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 Influenced by Pasteur’s notion that microorganisms cause disease he began treating his
surgical equipment with chemicals to kill any microbes that may be present
 Death from infection rates in the hospital he worked in dropped significantly after he
started this practice
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Robert Koch
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 This methodology (known as Koch’s Postulates) is still used today (see handout)
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Disease Transmission
 Infectious disease can be transmitted by one of the following ways:
1.
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31.2 The Body’s defense
 The Human body has three lines of defense against infectious disease:
1. Barriers - External Nonspecific Defenses
2. Internal Nonspecific Defenses
3.
1) Barriers (physical and chemical)
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 Sweat glands in skin secrete acids and oils which act as a chemical barrier to pathogens
 Sweat contains an enzyme (lysozyme) which breaks down cell walls of many bacteria
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 Your nose is protected by the fine hairs inside which are lined with mucus
 Mucus also is found in the digestive and respiratory pathways and is a sticky barrier which
germs often get stuck in
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2) Internal Nonspecific Defenses
a) Pathogen-destroying white blood cells
 Macrophages (big eater) – a large type of WBC that engulfs foreign bodies and
destroys them by phagocytosis…once inside the macrophage, enzymes are used
to kill the pathogen
 Neutrophils – smaller and more numerous than macrophages… like
macrophages neutrophils engulf invaders and release chemicals to kill them but
these chemicals kill the neutrophil as well
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b) Inflammatory response
 If a pathogen gets past the first line of defense it may trigger the inflammatory
response
 When the skin is broken, like in a cut, the broken tissue (mast cells) secretes
chemicals called histamines that cause vessels to expand (dilate)…this will increase
blood flow to the area and cause the swelling, redness, and heat associated with an
injury
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 If pathogens get into the blood stream and circulate throughout the body…your entire
body may trigger an inflammatory response (we call it a fever)
 A fever may be dangerous if it rages out of control…but a mild fever can help
stimulate more WBC’ s which can help destroy the pathogen
c) Specialized proteins
 Chemicals produced by cells in response to being infected by a virus
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 The infected cell may die but the specialized protein it produces will stimulate nearby cells
to do the same…a large number of cells together may produce enough of the specialized
protein to destroy the viruses or block their reproduction…ex. – Interferon
3) Targeted Defense
 The third line of defense against infectious disease is your body’s immune system
 The immune system targets specific pathogens, cancer cells, and certain chemicals
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 The attack may take longer but is longer lasting and more effective
31.3 The Immune System
Antigens
 The surfaces of viruses, bacteria, fungi, cancer cells, pollen and other cells contain certain
molecules called antigens
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 Antigen means “antibody-generating”
 Each pathogen’s antigen has it’s own unique shape
Antibodies
 Antibodies are proteins that attach to particular antigens
 Antibodies are found on the surface of certain white blood cells or in blood plasma
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 These binding sites shapes fit specifically to certain antigens that are the same shape (like
two pieces of a puzzle)
 Antibodies “tag” invaders by binding to the antigen molecules
 Antibodies neutralize or destroy invaders in a variety of ways:
a. it could prevent a virus from entering a host cell
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c. it could cause pathogens to clump together making them an easy target for phagocytes
 All of these responses involve a specific recognition followed by a non-specific
destruction
Lymphocytes
 This is the name given to white blood cells that recognize specific invaders
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 Two types of lymphocytes are B and T cells
 B and T cells differ from one another and amongst themselves as well
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Humoral Immunity
 Provided by B cells
 B cells provide immunity and protection outside of cells in body fluids
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 All the plasma cells will produce the same antibody (the one that works) until the invader
is destroyed or stopped
Cell-Mediated immunity
 Provided by T cells
 T cells operate by attacking infected body cells
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 If a T cell has a matching receptor site then it activates and produces a clone army
(cytotoxic T cells…aka killer T cellls)
 The army will bind to other infected cells and poke holes in them by producing a chemical
called perforin
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 T cells are also critical to the “in house” prevention of cancer
 T cells can kill cancer cells in the same way that they destroy infected body cells
 Why this system sometimes fails is a critical aspect of cancer research
Helper T cells
 Helper T cells help boost both humoral and cell mediated immunity
 Like other WBC’ s, helper T cells have binding sites that bind to known antigens
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 Once a pathogen antigen is identified on a macrophage the helper T cells begin secreting
chemicals that call cytotoxic T cells and B cells into action
Primary Immunity
 The first time that T or B cells battle a new pathogen it is known as the primary
immune response
 The primary response is slow and weak because time is needed for the proper cell with
the correct binding site to be found as well as the tome needed to generate a clone army
large enough to defeat the invader
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 These memory cells will be used if there is ever a repeat visit by the intruder
Secondary Immunity
 After the primary immune response, the memory T or B cells that remain in the body are
poised to spring into action immediately if the pathogen ever returns
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 This stronger and quicker response to infection is called the secondary response
31.4 Vaccinations
Vaccines and natural immunity
 Natural immunity is gained after being infected by a pathogen or receiving a vaccination
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 A vaccine is a dose of a weakened or inactive pathogen...or part of a pathogen’s antigen
that stimulates the primary immune response
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What is a vaccine made of?
 Some vaccines are made of only the pathogen’s antigen
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 Some vaccines consist of a slightly different version of the pathogen that “fools” the body
into reacting (EX. Cowpox for Smallpox)
 Booster shots are given for some vaccines to boost antibody production and strengthen the
memory cells that were created after the first exposure
Active Immunity
 Active immunity is developed whenever a body produces it’s own antibodies against an
infection
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Passive Immunity
 Receiving already synthesized antibodies gives us passive immunity
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 Passive immunity provides only temporary protection against infection because the
antibodies will disintegrate over time…and no memory cells will be produced…remember
only the primary immune response can provide long term protection
31.5 Immune system disorders
Allergies
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 Most allergens stimulate mast cells to produce histamine
 Histamine triggers the inflammatory response, which produces the characteristic
symptoms of watery eyes, runny nose, sneezing, etc.
 Some allergies may be life threatening if serious enough to cause a sudden dramatic drop
in blood pressure and a breathing arrest – a condition known as anaphylactic shock
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Autoimmune Disease
 Occurs when the bodies immune system turns against it’s own molecules (can’t
distinguish “self” from “non-self”)
 Examples:
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b. Rheumatoid arthritis – the immune system attacks cartilage and bone joints causing
damage and painful debilitating inflammation
c. Multiple Sclerosis- T cells attack parts of nerve cells and interfere with the
transmission of nerve signals – symptoms range from fatigue and numbness to
difficulties with walking, talking, concentrating, and even memory loss
 Autoimmune disorders are treated by either replacing lost “self” molecules, suppressing
the immune system, or just treating the symptoms themselves
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AIDS – A Disease of the Immune System
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Stands for Acquired Immune Deficiency Syndrome
AIDS is caused by HIV – Human Immunodeficiency Virus
HIV attacks primarily the helper T cells
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Both humoral and cell mediated immunity are affected…leaving the body open to
infections such as pneumonia or death from certain forms of cancer
 AIDS is transmitted primarily through sexual contact or through contaminated blood via
shared drug needles
 Because HIV is a retrovirus it mutates rapidly…therefore no vaccine can be produced
Organ Transplants
 Transplanted organs are often viewed as “non-self” by the bodies immune system and are
attacked
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 These drugs however leave the body vulnerable to infection from other sources
 Transplants that have the fewest complications are those between identical twins
Healthy Immune system
 A healthy Immune system is supported by a balanced diet…good hygiene…good
sleep…and an avoidance of stress
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