31.1 Infectious disease caused by pathogens
Germ theory
 Germ theory states that infectious disease is caused by pathogens
 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
 Pasteur became convinced that microorganisms caused the interference and
disease in humans
Joseph Lister
 Lister was a surgeon in England who was deeply concerned with the number
of needless deaths after surgery caused by infections
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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
 This was the beginnings of the first disinfectants and in his honor the mouth
wash Listerine is named
Robert Koch
 Robert Koch, who was a physician in Germany pioneered how to determine
which microorganism causes a specific disease
 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. Through the air
2. Direct contact with infected person or an article touched by an infected
person
3. Contaminated food or water
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4. Sexual contact
5. Animals or insect bites
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. Targeted Defense
1) Barriers (physical and chemical)
 Layer of dead skin on the surface acts as a physical barrier to pathogens unless
cut
 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
 Your mouth and eyes are also protected by lysozyme which is found in saliva
and tears
 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
 Stomach acids and enzymes help destroy most pathogens that get by these
other defenses
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
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 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
 Natural Killers – a white blood cell that kills abnormal cells as apposed to killing
pathogens…if a cell is infected with a virus or cancerous NK’ s will destroy
them…these cells are critical to a body’s cancer prevention
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
 Other chemicals produced by damaged cells attract WBC’ s which will attack
pathogens and clean up the damaged cells
 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
 These chemicals either attack invaders directly or block their reproduction
 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
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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
 The immune system will launch a customized attack for each of the
aforementioned targets
 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
 An antigen is usually a large protein molecule that provokes an immune response
 Antigen means “antibody-generating”
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 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
 Antibodies are “Y” shaped molecules that include an antigen binding site at the
tip of each end
 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
b. it could bind to the toxin producing part of a bacteria and prevent
further production
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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 Lymphocytes originate in bone marrow and when mature travel throughout the
blood and lymph system
 Two types of lymphocytes are B and T cells
 B and T cells differ from one another and amongst themselves as well
 During development, many different shaped antigen receptors are generated in
these lymphocytes (about 100 million variations in B cells alone!)
Humoral Immunity
 Provided by B cells
 B cells provide immunity and protection outside of cells in body fluids
 If a B cell’s binding site matches a particular antigen then that cell will grow and
begin to produce a clone army (millions) of plasma cells
 All the plasma cells will produce the same antibody (the one that works) until the
invader is destroyed or stopped
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Cell-Mediated immunity
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Provided by T cells
T cells operate by attacking infected body cells
Infected cells display the pathogens antigen on it’s surface
If a T cell has a matching receptor site then it activates and produces a clone
army (cytotoxic T cells…aka killer T cells)
The army will bind to other infected cells and poke holes in them by producing a
chemical called perforin
By destroying the cells the cytotoxic T cells stop viral reproduction
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
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 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
 Helper T cells bind to Macrophages which display pathogen antigens on their
surface
 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 time needed to
generate a clone army large enough to defeat the invader
 After the primary immune response has ended, special long-lasting lymphocytes
called memory cells remain in your body
 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
 The reaction of these memory cells is so fast that you never even develop the
symptoms of the disease
 This stronger and quicker response to infection is called the secondary response
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31.4 Vaccinations
Vaccines and natural immunity
 Natural immunity is gained after being infected by a pathogen or receiving a
vaccination
 Vaccines are given for diseases that could be life threatening
 A vaccine is a dose of a weakened or inactive pathogen...or part of a pathogen’s
antigen that stimulates the primary immune response
 Memory cells that are created by the primary immune response give long lasting
immunity to the disease (natural immunity)
What is a vaccine made of ?
 Some vaccines are made of only the pathogen’s antigen
 Some vaccines are created by grinding up or heating a dangerous pathogen so
that it is no longer functional
 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
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Active Immunity
 Active immunity is developed whenever a body produces it’s own antibodies
against an infection
 Active immunity follows any infection or vaccination
Passive Immunity
 Receiving already synthesized antibodies gives us passive immunity
 Antibodies can be received by injection or even while in the womb (from our
mother)
 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
 An allergic reaction is caused by the immune systems response to an
otherwise non-harmful antigen (allergen)
 Most allergens stimulate mast cells to produce histamine
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 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
 People with sever allergies often carry with them a syringe full of epinephrine
that counteracts the allergies symptoms
Autoimmune Disease
 Occurs when the bodies immune system turns against it’s own molecules
(can’t distinguish “self” from “non-self”)
 Examples:
a. Lupus – the body makes antibodies that may react with any of the bodies
tissues – creates a variety of symptoms depending on which tissues are
attacked
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
Without the helper T cells the immune system can not activate other T or B
cells
 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
 Immune system suppressing drugs must be prescribed to stop this attack
known as “rejection”
 These drugs however leave the body vulnerable to infection from other
sources
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 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
 Stress can cause the production of chemicals that suppress the immune system
and make you more vulnerable to disease
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