6.3 Defense Against Infectious
Disease
The human body has structures and
processes that resist the continuous
threat of invasion by pathogens.
Q: What is a pathogen?
Bacteria
• Prokaryotes (no real nucleus)
• Divide by binary fission
Can cause:
• Food poisoning (e.g. Salmonella)
• Ear and eye infections
http://en.wikipedia.org/wiki/File:Ericson_Type
• Cholera, diarrhea
_II_Conjunctivitis.JPG
Viruses
• Acellular (non-living?)
• Need a ‘host’ cell to carry out functions of life, including reproduction
• Can have DNA or RNA
• Mutate, evolve and recombine quickly
Cause:
• Flu, HIV/AIDS, smallpox, measles, common cold, herpes, ebola
The 1918 flu epidemic killed between 50 and 130 million
people. http://en.wikipedia.org/wiki/1918_flu_pandemic
Fungi
• Eukaryotes, reproduce with spores
Cause:
• Athlete’s foot, mould, ringworm
• Allergic reactions and respiratory
problems
Image from:
http://en.wikipedia.org/wiki/Athlete's_foot
Protozoa
• Simple parasites
Cause:
• Malaria
• Leishmaniasis
• Toxoplasmosis
Leishmaniasis image from:
http://en.wikipedia.org/wiki/Leishmaniasis
The skin and mucous membranes form a primary
defence against pathogens that cause infectious
disease.
Q: What parts of the
body have mucous
membranes?
Q: List two
characteristics of mucous
membranes that help
them defend against
pathogens.
Cuts in the skin are sealed by blood
clotting.
Q: In numbered steps, explain how clotting
occurs.
• Clotting factors are released from platelets.
• An enzyme called thrombin is produced.
• Thrombin converts soluble fibrinogen to
insoluble fibrin.
• Fibrin forms a mesh that traps red blood cells.
• The gel, if exposed to air, hardens into a scab.
Application: Causes and consequences of
blood clot formation in coronary arteries.
CAUSES
CONSEQUENCES
Well known factors correlated
with an increased risk of blood
clot formation in coronary
arteries.
• Smoking
• High blood cholesterol
concentration
• High blood pressure
• Diabetes
• Obesity
• Lack of exercise
If the coronary arteries become
blocked by a blood clot:
• Part of the heart is deprived of
O2 and nutrients
• Cardiac muscle cells can not
produce ATP
• Contractions become irregular
and uncoordinated
• Fibrillation
• Heart does not pump blood
effectively
Review atherosclerosis
Ingestion of pathogens by phago
cytic white
blood cells gives non-specific immunity to diseases.
(“eating”)
(“cell”)
Q: Draw the process of phagocytosis. Why is this “non-specific immunity”?
• Chemotaxis (movement in response to chemicals) attracts the phagocytes
to the area of invasion as response to:
• proteins produced by the pathogen
• phospholipids released by damaged cells
• The phagocyte attaches to the pathogen’s cell surface proteins and then
engulfs it. The fluid nature of the plasma membrane allows this to happen.
• A phagosome forms. This is a vesicle that contains the pathogen.
Lysosomes – vesicles of digestive enzymes – deposit the enzymes into the
phagosome.
• The digestive enzymes break down the pathogen and the waste products
are expelled from the cell by exocytosis.
Q: Why do infected wounds become pus-filled?
Phagocytic Leucocytes
card sort game: order the images
& outline the processes
Images from: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__phagocytosis.html
Q: What is a lymphocyte?
Q: What is an antibody?
• Production of antibodies by lymphocytes in
response to particular pathogens gives specific
immunity.
Some lymphocytes act as memory cells and can quickly
reproduce to form a clone of plasma cells if a pathogen carrying
a specific antigen is re-encountered.
Q: Why does it take a couple of days for your
body to be able to fight off a disease?
Antibiotics block processes that occur in
prokaryotic cells but not in eukaryotic cells.
Q: List four processes in prokaryotes that are
blocked by antibiotics.
Viruses lack a metabolism and cannot
therefore be treated with antibiotics.
Antibiotics are ineffective against viruses!
Analyse the graph below. Over time, outline what has happened to:
• The number of new approved antibiotics
• The diversity of new approved antibiotics
Some strains of bacteria have evolved with genes that
confer resistance to antibiotics and some strains of
bacteria have multiple resistance.
Q: List three measures that can be taken to reduce
the development of antibiotic resistance in bacteria.
Nature of science: Risks associated with scientific research—
Florey and Chain’s tests on the safety of penicillin would not be
compliant with current protocol on testing.
• Application: Florey and Chain’s experiments to test penicillin on
bacterial infections in mice.
• They tested the drug on humans after only a very brief period of
animal testing….
• How brief?
• 8 mice.
• There could have been severe side effects.
• The samples they were using were not pure.
• The patients they used were all at the point of death and several
were cured.
• Penicillin was introduced more quickly than it should have been, in
time for the D-Day invasion and the number of soldier deaths was
greatly reduced.
Application: An understanding of immunity has
led to the development of vaccinations.
• LIVE, ATTENUATED VACCINES
• Live, attenuated vaccines contain a version of the living microbe
that has been weakened in the lab so it can’t cause disease.
• Because a live, attenuated vaccine is the closest thing to a natural
infection, these vaccines are good “teachers” of the immune
system: They elicit strong cellular and antibody responses and often
confer lifelong immunity with only one or two doses.
• Vaccines against measles, mumps, and chickenpox, for example, are
made by this method. Viruses are simple microbes containing a
small number of genes, and scientists can therefore more readily
control their characteristics.
Application: An understanding of immunity has
led to the development of vaccinations.
• INACTIVATED VACCINES
• Scientists produce inactivated vaccines by killing the diseasecausing microbe with chemicals, heat, or radiation.
• Such vaccines are more stable and safer than live vaccines: The
dead microbes can’t mutate back to their disease-causing state.
Inactivated vaccines usually don’t require refrigeration, and they
can be easily stored and transported in a freeze-dried form, which
makes them accessible to people in developing countries.
• Most inactivated vaccines, however, stimulate a weaker immune
system response than do live vaccines.
• So it would likely take several additional doses, or booster shots, to
maintain a person’s immunity. This could be a drawback in areas
where people don’t have regular access to health care and can’t get
booster shots on time.
Application: An understanding of immunity has
led to the development of vaccinations.
• SUBUNIT VACCINES
• Instead of the entire microbe, subunit vaccines include only the antigens
that best stimulate the immune system.
• In some cases, these vaccines use epitopes—the very specific parts of the
antigen that antibodies or T cells recognize and bind to.
• Because subunit vaccines contain only the essential antigens and not all
the other molecules that make up the microbe, the chances of adverse
reactions to the vaccine are lower.
• A recombinant subunit vaccine has been made for the hepatitis B virus.
– Scientists inserted hepatitis B genes that code for important antigens into
common baker’s yeast.
– The yeast then produced the antigens, which the scientists collected and
purified for use in the vaccine.
– Research is continuing on a recombinant subunit vaccine against hepatitis C
virus.
Application: An understanding of immunity has
led to the development of vaccinations.
• TOXOID VACCINES
• For bacteria that secrete toxins, or harmful chemicals, a toxoid
vaccine might be the answer. These vaccines are used when a
bacterial toxin is the main cause of illness.
• Scientists have found that they can inactivate toxins by treating
them with formalin, a solution of formaldehyde and sterilized
water. Such “detoxified” toxins, called toxoids, are safe for use in
vaccines.
• When the immune system receives a vaccine containing a harmless
toxoid, it learns how to fight off the natural toxin. The immune
system produces antibodies that lock onto and block the toxin.
• Vaccines against diphtheria and tetanus are examples of toxoid
vaccines.
Application: An understanding of immunity has
led to the development of vaccinations.
• CONJUGATE VACCINES
• If a bacterium possesses an outer coating of sugar molecules called
polysaccharides, as many harmful bacteria do, researchers may try
making a conjugate vaccine for it.
• Polysaccharide coatings disguise a bacterium’s antigens so that the
immature immune systems of infants and younger children can’t
recognize or respond to them. Conjugate vaccines, a special type of
subunit vaccine, get around this problem.
• When making a conjugate vaccine, scientists link antigens or toxoids
from a microbe that an infant’s immune system can recognize to
the polysaccharides. The linkage helps the immature immune
system react to polysaccharide coatings and defend against the
disease-causing bacterium.
• The vaccine that protects against Haemophilus influenzae type B
(Hib) is a conjugate vaccine.
Application: An understanding of immunity has
led to the development of vaccinations.
•
•
•
•
•
•
DNA VACCINES
Still in the experimental stages, these vaccines show great promise, and several types are being
tested in humans. DNA vaccines take immunization to a new technological level.
These vaccines dispense with both the whole organism and its parts and get right down to the
essentials: the microbe’s genetic material. In particular, DNA vaccines use the genes that code for
those all-important antigens.
A DNA vaccine against a microbe would evoke a strong antibody response to the free-floating
antigen secreted by cells, and the vaccine also would stimulate a strong cellular response against
the microbial antigens displayed on cell surfaces.
The DNA vaccine couldn’t cause the disease because it wouldn’t contain the microbe, just copies of
a few of its genes. In addition, DNA vaccines are relatively easy and inexpensive to design and
produce.
So-called naked DNA vaccines consist of DNA that is administered directly into the body. These
vaccines can be administered with a needle and syringe or with a needle-less device that uses highpressure gas to shoot microscopic gold particles coated with DNA directly into cells. Sometimes, the
DNA is mixed with molecules that facilitate its uptake by the body’s cells. Naked DNA vaccines being
tested in humans include those against the viruses that cause influenza and herpes.
Application: An understanding of immunity has
led to the development of vaccinations.
•
•
•
•
•
RECOMBINANT VECTOR VACCINES
Recombinant vector vaccines are experimental vaccines similar to DNA vaccines,
but they use an attenuated virus or bacterium to introduce microbial DNA to cells
of the body. “Vector” refers to the virus or bacterium used as the carrier.
In nature, viruses latch on to cells and inject their genetic material into them. In
the lab, scientists have taken advantage of this process. They have figured out how
to take the roomy genomes of certain harmless or attenuated viruses and insert
portions of the genetic material from other microbes into them. The carrier viruses
then ferry that microbial DNA to cells. Recombinant vector vaccines closely mimic
a natural infection and therefore do a good job of stimulating the immune system.
Attenuated bacteria also can be used as vectors. In this case, the inserted genetic
material causes the bacteria to display the antigens of other microbes on its
surface. In effect, the harmless bacterium mimics a harmful microbe, provoking an
immune response.
Researchers are working on both bacterial and viral-based recombinant vector
vaccines for HIV, rabies, and measles.
Application: Effects of HIV on the immune system (a reduction
in the number of active lymphocytes and a loss of the ability to
produce antibodies, leading to the development of AIDS) and
methods of transmission.