Topic 6.3
Defence against
infectious disease
Topic 6: Human Health and Physiology
What is a pathogen?
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Patho = disease Gen = Producer
A pathogen is an organism or virus that
causes a disease.
Why aren’t viruses called organisms??
Organisms are living things. What are the
characteristics of living things?
Characteristics of living things
Properties of life
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Cellular Respiration
Reproduction
Metabolism
Homeostasis
Heredity
Responsiveness
Growth and development
Viruses infecting living cells
Viruses are not living organisms
Viruses do not
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Grow
Have homeostasis
Metabolize
Viruses do
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Infect cells and use the cell to make more
viruses
Cause disease in many organisms
Viruses
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Viruses are micro organisms consisting of
a strand of DNA or RNA surrounded by a
protein coat
They enter cells and hijack the cells
machinery to make more viruses.
They then burst out of the host cell,
destroying or damaging it.
Other pathogens
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Bacteria
Fungi
Protozoa
Animals
Prions
Antibiotics
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Major medical improvement
Produced by fungi and bacteria
Work on bacteria but…
Cannot be used on viruses
antibiotics block specific metabolic pathways found in bacteria, but not eukaryotic cells
because viruses reproduce using the host cell (eukaryotic) metabolic pathways, they are
unaffected by antibiotics
antibiotics have produced great benefits world-wide in the control of bacterial diseases
 Staphylococcus infections controlled
 STD's, such as gonorrhea and syphilis controlled
antibiotic resistance has evolved in bacterial populations
Non specific immunity
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The body resists infection with general
(non-specific) and specific methods.
Non-specific immunity stops a wide variety
of pathogens.
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Skin
Mucous membranes
Tears
Sweat
Hair
Sebum
Blood clots
Cilia
Stomach acid
Skin
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Unbroken skin
provides a fantastic
barrier against
pathogens trying to
enter the body
Sebum (oil)
waterproofing
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You do not need to
learn the parts of this
diagram.
Mucous Membranes
(and other non-specific immunity)
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Anywhere on the body that is not protected by
skin has its own method of protection.
Lungs – Mucous
membranes and cilia
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Stomach – Acid
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Urethra – mucus
membranes + urine is
sterile
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Eyes – tears contain
lysozymes
Vagina – mucous
membranes and acid
Anus – mucous
membranes
Infection!
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This occurs if pathogens do get inside the
body.
The proteins on the surface of a pathogen
are immediately recognised as “foreign”.
Phagocytes (a type of leucocytes) will
ingest the pathogen by phagocytosis.
11.1.1 Blood Clotting HL
Clotting is the mechanism that prevents and blood loss
from broken blood vessels.
Mechanism
a) Platelets or damaged cells release a group of proteins
called clotting factors. These clotting factors are released
into the plasma a wound site.
b) Clotting factors activate the enzyme Thrombin from
its inactive form prothrombin
c) Thrombin turns the soluble plasma protein fibrinogen
into its insoluble fibrous form Fibrin.
d) Fibrin binds together platelets and blood cells to form
a solid 'plug' for the4 wound. This plug is called a clot.
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http://biology-animations.blogspot.com/2008/01/hemophilia-video.html
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http://biology-animations.blogspot.com/2008/01/for-detailed-information-httpen.html
Phagocytosis SL
Phagocytes
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Phagocytes are found
in the blood and in
body tissues such as
the lungs.
After phagocytes
engulf pathogens,
they destroy them
with digestive
enzymes found in
lysosomes.
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damage to tissues allows invasion across 1st line of
defense
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phagocytes attracted to site by chemotaxis toward
histamine
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microbes successfully invade body fluids or tissues
damaged cells release histamine and other chemicals initiating
inflammation
phagocytes recognize microbes as foreign by antigen recognition
variety of phagocytic cells: neutrophils (small phagocytic &
macrophages (large phagocytic)
phagocytes endocytotically engulf microbes,which are
digested by enzymes held in lysosomes
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digested microbe fragments are displayed on cell membrane
phagocytes with microbe fragments displayed = antigenpresenting cells
Phagocytosis
Phagocytosis
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This animation is in too much detail for SL
IB but it shows the process perfectly!
6.3.5 Distinguish between
antigens and antibodies SL
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antigen: a molecule recognized as foreign by the immune system; it
elicits an immune response
antibody: =immunoglobulin
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a globular protein
recognizes an antigen by its complementary shape and charge
thus allowing it to attach to the antigen specifically
marking it for attack by the immune system
Antigens
Anything that
elicits an
immune
response
 Usually a
foreign protein
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A cat and its dander, an
allergen/antigen for many
people
Antibodies SL
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Also known as immunoglobulins
Globular glycoproteins
The heavy and light chains are polypeptides
The chains are held together by disulphide
bridges
Each antiboby has 2 identical antigen binding
sites – variable regions.
The order of amino acids in the variable
region determines the shape of the binding
site
Antigens
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Most are proteins or large polysaccharides from a foreign organism.
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Microbes: Capsules, cell walls, toxins, viral capsids, flagella, etc.
Nonmicrobes: Pollen, egg white , red blood cell surface molecules,
serum proteins, and surface molecules from transplanted tissue.
Lipids and nucleic acids are only antigenic when combined with proteins or
polysaccharides.
HL Challenge and response
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Challenge:
antigens:
 substances foreign to the host
 which stimulate antibody production by B lymphocytes
Response:
antibodies:
 immunoglobulins
 produced during protein synthesis
How Antibodies work?
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Some act as labels to identify
antigens for phagocytes
Some work as antitoxins i.e. they block
toxins for e.g. those causing diphtheria and
tetanus
Some attach to bacterial flagella making
them less active and easier for phagocytes
to engulf
Some cause agglutination (clumping
together) of bacteria making them less likely
to spread
Type
Number of Site of action
ag binding
sites
Functions
IgG
2
Increase
IgM
10
Blood
Tissue
fluid
CAN CROSS
PLACENTA
macrophage activity
Antitoxins
Agglutination
Blood
Agglutination
Tissue
fluid
IgA
2 or 4
Secretions
(saliva,
tears, small intestine,
vaginal, prostate,
nasal, breast milk)
Stop
bacteria
adhering to host
cells
Prevents bacteria
forming colonies on
mucous membranes
IgE
2
Tissues
Activate
mast cells
 HISTAMINE
Worm response
Antibodies are actively released
by lymphocytes when the body
is infected by a pathogen. This
is a natural process.
The lymphocytes are activated by the
antigen on the pathogens outer surface.
HL- A vaccine can also be artificially injected
into the body. Advantages/
Disadvantages?
Blood types
HL: To Vaccinate or not?
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Vaccines:
MMR = combined measles/mumps/rubella
DPT = diphtheria, pertussis, typhoid
polio
Benefits:
eradication of some diseases (e.g. small pox)
reduced rate of infection; less illness and death
reduced long-term disability
Dangers:
low percentage of vaccinations produce side effects, including symptoms of disease,
sometimes serious or lethal
artificial immunity not as effective at producing immune response as is active
immunity
excessive vaccination may reduce the ability of the immune system to respond to
new diseases
vaccination possibly implicated in autism
Active vs. Passive Immunity
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Active immunity: immunity acquired
through individual immune response
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Catching a disease
Passive immunity: immunity acquired
through antibodies transferred from
one person to another
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Mother’s milk
HL: Active and passive
immunity
However, it takes time for the body to react to the infection
and produce white (B and T) cells. During this time the
disease can multiply and spread rapidly.
Tetanus is an especially deadly disease that
can kill in hours. Antibodies can be
collected from people who have already
been vaccinated against it. These can then be injected
into people who may have caught the infection. Its only
a temporary fix- the spleen and liver break them down.
This is Passive Immunity.
Mothers pass antibodies through the placenta
during gestation. Colostrum is a liquid made
in the mammary glands during the first few
days after birth- it lines the gut and protects
against bacteria and viruses.
HL Where do antibodies come
from?= Lymphocytes
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Produce antibodies
B-cells mature in bone marrow then
concentrate in lymph nodes and spleen
T-cells mature in thymus
B and T cells mature then circulate in the
blood and lymph
Circulation ensures they come into contact
with pathogens and each other
HL White Blood cells
WBC (Lymphocytes) are made in the bone marrow. The
cells divide and mature into 2 types:
T Lymphocytes
Develop in the thymus gland
The Cell-Mediated Resopnse
Foreign, mutant or infected cells are
identified as ‘non-self’ by surface antigens.
T helper cell recognises antigens and
alerts the other T cells.
Killer (Cytotoxic) cells attack with perforin
or nitric oxide. T memory cells remember
the antigen for a later attack.
T suppressor cells switch off T and B cells
after attack over.
B Lymphocytes
Develop in the lymph nodes
The Humoral (Antibody) Response
Plasma ɞ cells have a different shape
receptor on its membrane. There are about
10 million kinds in your body at any one
time! Each can detect any antigen of a
pathogen. Once activated they divide very
fast (clone) and start to secrete specific
antibodies. Antibodies slowly removed
from blood and lymph. Memory ɞ cells
remember antigens and can turn into
Plasma ɞ if re-infected.
How Do B Cells Produce Antibodies?
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B cells develop from stem cells in the
bone marrow of adults (liver of fetuses).
After maturation B cells migrate to
lymphoid organs (lymph node or spleen).
Clonal Selection: When a B cell
encounters an antigen it recognizes, it is
stimulated and divides into many clones
called plasma cells, which actively secrete
antibodies.
Each B cell produces antibodies that will
recognize only one antigenic determinant.
B -Lymphocytes
Challenged and Response
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The antibodies travel
to the blood, lymph,
lining of gut and
lungs.
The number of
plasma cells goes
down after a few
weeks
Antibodies stay in the
blood longer but
eventually their
numbers go down
too
Clonal Selection of B Cells is
Caused by Antigenic Stimulation
Duality of Immune System
I. Humoral (Antibody-Mediated) Immunity
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Involves production of antibodies against foreign
antigens.
Antibodies are produced by a subset of lymphocytes
called B cells.
B cells that are stimulated will actively secrete
antibodies and are called plasma cells.
Antibodies are found in extracellular fluids (blood
plasma, lymph, mucus, etc.) and the surface of B
cells.
Defense against bacteria, bacterial toxins, and
viruses that circulate freely in body fluids, before
they enter cells.
Also cause certain reactions against transplanted
tissue.
Immunological Memory
Secondary Response:
 Subsequent exposure to the same antigen displays a faster
and more intense antibody response.
 Increased antibody response is due to the existence of
memory cells, which rapidly produce plasma cells upon
antigen stimulation.
 Booster shots increase the number of memory cells
HL Production of monoclonal
antibodies
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Extract B cell
that can produce
the required
antibodies
Hybridise with a
tumour cell
(capable of
endless division)
In culture the
hybridoma can
produce large
quantities of
antibodies
Production of monoclonal antibodies
and their use
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Use in diagnosis – will be covered in 11.4.10 (pregnancy diagnostic)
Treatment
Diagnosis: HIV infection can be diagnosed through a blood test
which assays for the presence of anti-HIV antibodies,
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antibody produced through monoclonal antibody production is an antianti-HIV antibody!
Treatment:
Rabies Virus
monoclonal antibodies can be produced which selectively locate and
adhere to cancer cells
anti-cancer drugs can be attached to the monoclonal antibodies
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so that they deliver their effects directly to the targeted cancer cells
Immune System Abnormalities
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Allergies:
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Exaggerated immune response to
antigens called allergens
Autoimmune diseases:
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Body doesn’t recognize “self” and
mounts immune response
HIV
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HIV is a virus that specifically attacks the
lymphocytes.
This means the number of lymphocytes
decreases.
Less antibodies are made.
Predict the consequences…
Click here for a complex video on how the
virus enters the lymphocyte
Transmission of HIV
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You should know this from IGCSE! You
tell me!!
Transmission of HIV
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Infected blood – blood transfusions,
sharing needles,
Infected semen and vaginal mucus –
unprotected sex
Infected mother’s milk – low risk
Infected saliva – almost zero risk
AIDS
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Acquired Immuno-Deficiency Syndrome
Caused by the HIV virus that selectively
infects the immune system leaving the
body open to infection by removing the
specific immunity.
HIV Progression
Social implications of AIDS
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The social implications of AIDS are well known.
Cases of AIDS are not evenly distributed in the
world for example there are severe problems in
southern Africa.
What cultural and economic reasons are there
for differences in the prevalence of AIDS?
Is there a moral obligation of those with the
technology and the wealth to help others lacking
these things in the fight against AIDS?
Social implications of AIDS
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Homework:
Read the article “AIDS takes an economic
and Social toll”.
Highlight the key points.
Read P103 in your text book
Make notes on the key points.