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• Pathogens
• Microorganisms that cause diseases
• Commensals
• Those that live in harmony with the host
without causing any damage to it

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INFECTION
• Lodging and multiplication of pathogens in or
on tissues of a host

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Primary infection
• Infection that develops in an otherwise healthy
host

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Re - infection
• Infection by the same organism again in the
same host

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Secondary infection
• Infection in the host affected by primary
infection by a new organism

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Disease-causing organisms
Organisms that cause disease are called pathogens.
bacteria fungi
protozoa
virus
What are the four major types of pathogen?

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How do pathogens cause illness?
Toxins
Toxins are harmful substances produced by the pathogen
that poison the body’s tissue and enzymes.
Reproduction
A rise in the number of pathogens can damage a cell, even
causing it to burst. Some pathogens hijack resources that the
cell needs to survive.
Immune response
Sites of infection often
become swollen, sore
and hot as a result of
increased blood flow.
Pathogens cause illness in three main ways:

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How are pathogens spread?
Different pathogens have different transmission routes:
direct contact indirect contact
food and water airborne droplets
insect bites

10. Immunity to
Infection

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Fighting infection
How does the body fight infection?

12. Immunity to Infection
• Immunity is the acquired ability to defend
against infection by disease-causing
organisms.

13. Overview of your immune system
• First line of defense: Physical barriers that viruses, bacteria must cross
– skin covers ~2 m2
– Mucous membranes that line digestive, respiratory, reproductive tracts cover
~400 m2
• Second line of defense: Innate immune system (germline-encoded
receptors -- no adaptation to specific pathogens)
– Macrophages (Greek for “big eater”), neutrophils, natural killer (NK) cells
• Third line of defense (vertebrates only): Adaptive immune system
(adapts to defend against specific pathogens using variable receptors)
– B cells make antibodies that vary -- can make an antibody specific for any
new antigen
– T cells mediate cellular responses using variable receptors (T cell receptors;
TCRs)

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First line of defence

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Painkillers can relieve the symptoms of an infection but do
not kill the pathogen.
Invasion!
A pathogen enters your body through direct or indirect
contact. What happens next?
Your immune system begins to mount an attack.
The pathogen begins to
reproduce and make toxins that destroy
the body’s cells and make you feel unwell.

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Second line of defence – Innate immunity
Macrophages

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Second line of defence - Natural killer (NK) cells
• Can kill tumor cells, virus-infected cells, bacteria, parasites, fungi in
tissues
• Identify targets based on “missing self”
– Two types of NK receptors: inhibitory and activating
– If inhibitory receptor recognizes a self protein (a class I MHC molecule) on a target
cell, the NK cell is turned OFF even if activating receptor binds a ligand on the
same target cell
– If activating receptor binds a ligand, but inhibitory receptor does not (target cell has
down-regulated class I MHC proteins), NK cells kill
– Many virally-infected cells and tumor cells down-regulate expression of class I
MHC molecules (NK cells important for preventing cancers)

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Resisting attack
TB bacteria have a number of adaptations that enable them
to resist phagocytosis:
 They produce chemicals that prevent macrophages from
being attracted to sites of infection.
 They have a waxy cell wall that resists enzyme attack
and secretes chemicals that block lysosomes from fusing
with phagosomes.
How does the immune system deal with these bacteria?

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Third line of defence - Lymphocytes
Lymphocytes are a type of white blood cell found in the
blood or lymph nodes and made by bone marrow. There are
several types of lymphocyte, including:
 T-lymphocytes – recognise antigens
on pathogens and either attack them
directly or co-ordinate the activity of
other cells of the immune system.
 B-lymphocytes – recognise
antigens and produce special
chemicals called antibodies.

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B lymphocytes
Antibodies are special
Y-shaped proteins
produced by
B-lymphocytes in
response to antigens.
Antibodies work by binding to antigens on pathogens,
‘labelling’ them and causing them to clump together.
The pathogen can then be destroyed by:
 the antibodies themselves.
 phagocytosis by macrophages
 T-lymphocytes

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Antibodies
Each different type of antigen causes a different type of
antibody to be produced.
An antibody can only bind to the antigen that caused it to be
produced.

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Delayed response
The B-lymphocyte that produces the correct antibody for the
antigen begins dividing to produce many more antibody-
producing cells.
It takes a few days to produce enough
antibodies to destroy the pathogen. This
means there is delay between infection
and the person beginning to feel better.
Once a pathogen has been destroyed, a few memory cells
remain. These recognize the pathogen if it re-infects, and
make the immune response much quicker and more effective.
This is called active immunity.

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Antibody levels during infection

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T cells
How does the body deal with pathogens that are inside cells?
Viruses and bacteria that infect cells leave antigens on the
surface of the cell they infect.
T-lymphocytes recognize these antigens by receptors on their
surface and destroy the whole infected cell.
infected cell
T-cell
antigen receptor

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Types of Immunity
Artificially acquired
Passive immunityActive immunity
Naturally acquired Naturally acquired
Artificially acquired

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Types of Immunity
• Natural immunity is the result of a body’s previous
encounter with an organism.
• Artificial immunity results from the injection of a
vaccine or an antibody. Vaccines stimulate active
immunity whereas injection antibody or antiserum is
an example of passive immunity.

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Types of Immunity
• Active immunity is when the immune system
encounters and antigen and is primed to recognise it
and destroy it quickly the next time it is encountered.
This is active immunity because the body’s immune
system prepares itself for future challenges.
• Passive immunity is short-term and involves the
transfer of immunity from one individual to
another via antibody-rich serum. This may be
artificial as is the case with anti-venom or natural, as
in antibodies crossing the placenta to protect the
developing foetus.

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Immunity
Active immunity
Production of a person’s own
antibodies. Long lasting
Passive immunity
An individual is given antibodies by another
Short-term resistance (weeks- 6months)
Natural Active
When pathogen
enters body in the
normal way, we
make antibodies
Natural Passive
Baby in utero
(placenta)
Breast-fed babies
Artificial Passive
Gamma globulin
injection
Extremely fast, but
short lived (e.g. snake
venom)
Edward Jenner
Artificial Active
Vaccination – usually
contains a safe antigen
from the pathogen.
Person makes
antibodies without
becoming ill

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Passive immunity
Many snakes produce a
powerful nerve toxin that
can be lethal to humans.
Antivenin contains antibodies to give instant immunity. It is
produced by injecting horses with small, non-lethal doses of
venom. Over time, the horses produce antibodies, which are
extracted and processed.
People bitten by
poisonous snakes can be
treated with antivenin.
Because the person didn’t make the antibodies themselves,
this is called passive immunity.

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Vaccines contain a small amount of dead or weakened
pathogen particles.
What are vaccines?
A vaccine stimulates the production of
antibodies and memory cells against the
target pathogen, without making the
person ill.
If a vaccinated person is later infected
by the same pathogen, their immune
system can destroy it very quickly.
Parents of two-year-old children are offered a combined
measles, mumps and rubella (MMR) vaccine to protect their
child. What has happened to MMR vaccination rates recently?

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Measles, mumps and rubella
MMR vaccination rates used
to be high, but fell following a
media scare story.
The media reported on
controversial research
speculating that MMR
could cause autism, a
behavioural disorder
causing learning and
communication difficulties.

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Glossary (1/2)
antibiotic – A drug that can destroy or prevent the growth
of bacteria.
antibody – A Y-shaped protein produced by the body that
binds to antigens.
antigen – A substance on pathogens that stimulates the
production of antibodies.
B-lymphocyte – A white blood cell that produces
antibodies.
immunization – The process of protecting against
infection by using a vaccine.
immune response – The body’s defence against foreign
material such as pathogens.

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Glossary (2/2)
immunity – The ability to fight infection by pathogens. It
can be active or passive.
pathogen – A disease-causing micro-organism.
phagocytosis – The process where a type of white blood
cell called a macrophage ingests and destroys a pathogen.
T-lymphocyte – A type of white blood cell that recognises
and destroys pathogens, and co-ordinates the immune
response.
tuberculosis – A serious bacterial disease that mainly
affects the respiratory system.
vaccine – A small amount of dead or weakened pathogen
that stimulates antibody production.

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Multiple-choice quiz

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