Download Active Immunisation and Vaccination

Unit 4 - Immunology and Public Health
CfE Higher Human Biology
24. Active Immunisation and Vaccination and
the Evasion of Specific Immune Response by
Pathogens
Learning Intentions
I can describe the importance of herd immunity in infectious disease control.
I can state that the herd immunity threshold depends on the disease, the efficacy
of the vaccine and the contact parameters for the population.
I can describe public health immunisation programmes.
I can state that establishing herd immunity to a number of diseases. Difficulties
when widespread vaccination is not possible due to malnutrition, poverty or vaccine
rejected by a percentage of the population.
I can describe the evasion of specific immune responses by pathogens.
I can state that many pathogens have evolved mechanisms that evade the specific
immune system which has consequences for vaccination strategies.
Antigenic variation.
I can state that some pathogens can change their antigens avoiding the effect of
immunological memory. Role and impact in diseases like malaria, trypanosomiasis and
influenza.
I can describe a direct attack on the immune system.
I can state that HIV attacks lymphocytes which are the major cause of AIDS.
I can state that Tuberculosis (TB) survives within phagocytes and avoids immune
detection.
Herd Immunity
• Vaccines can prevent outbreaks of
disease and save lives.
• When a critical portion of a
community is immunized against a
contagious disease, most members of
the community are protected against
that disease because there is little
opportunity for an outbreak. This is
because their chance of coming into
contact with someone with the
disease is minimal.
Even those who are not eligible for
certain vaccines—such as infants,
pregnant women, or
immunocompromised individuals —
get some protection because the
spread of contagious disease is
contained.
• This is known as “herd immunity."
•
Herd Immunity
• Box 1 shows a
community in which
no one is immunized
and an outbreak
occurs.
• Box 2, some of the
population is
immunized but not
enough to confer
herd immunity.
• Bow 3, a critical
portion of the
population is
immunized,
protecting most
community
members.
Watch the following video on the effect of adverse publicity on the MMR vaccination
http://www.youtube.com/watch?v=jfheO9H8CD4
Absence of herd immunity
Some populations do not have herd
immunity and this may be because of
very different reasons;
• In developing countries, malnutrition
and poverty prevent mass vaccination
programmes
• In developed countries, adverse
publicity about vaccines leads to
parents choosing to not have their child
vaccinated.
Herd immunity threshold
This is the percentage of the population who
need to be immunised by the vaccine to offer
protection for people who are not vaccinated.
Disease
Transmission
Average number of
secondary infections
resulting from single index
case
Diptheria
Saliva
6–7
85%
Measles
Airborne
12 – 18
83 – 94 %
Mumps
Airborne droplet
4–7
75 – 86 %
Whooping cough
Airborne droplet
12 – 17
92 – 94 %
Polio
Faecal-oral route
5–7
80 – 86 %
Rubella
Airborne droplet
5–7
83 – 85 %
Smallpox
Social contact
6–7
83 – 85 %
Herd immunity
threshold
Herd Immunity
• The principle of herd immunity applies
to control of a variety of contagious
diseases, including
– influenza,
– measles,
– mumps,
– rotavirus,
– pneumococcal disease.
Public Health Immunisation Programmes
• In most countries,
policy in public
health medicine is
to establish herd
immunity to a
number of
diseases.
Public Health Immunisation
Programmes
Public Health Immunisation Programmes
• Difficulties can arise when widespread
vaccination is not possible due to
– malnutrition and poverty (the developing
world),
Developed World
– Difficulties carrying out widespread vaccination
arise when large numbers of the population reject
immunisation programmes
– This this is a result of adverse publicity about
vaccinations
• eg Media attention and consequent public
concerns about vaccine safety followed
publication of a small case-series of children
who developed autism after receipt of the
measles-mumps-rubella (MMR) vaccine. Many
well-controlled studies performed subsequently
found no evidence that MMR vaccine causes
autism. However, despite these studies, some
parents remain concerned that the MMR vaccine
is not safe.
http://news.bbc.co.uk/1/hi/health/3041225.stm
Public Health Immunisation Programmes
• Because of vaccines, small pox is now
eradicated globally, polio nearly, and, in
countries where children are regularly
vaccinated, we don’t worry too much about
diphtheria, measles, whooping cough, and
rubella.
• Vaccination may be the most effective
public health intervention of all time—
that’s especially true in developing
countries, where many families can’t find
or afford health care when they get sick.
The prevention offered by vaccines can be
lifesaving.
Evasion Of Specific Immune Response
• Just as vertebrates have developed many
different defences against pathogens, so
pathogens have evolved elaborate strategies
to evade these defences.
• One way in which an infectious agent can
evade immune surveillance is by altering its
antigens; this is particularly important for
extracellular pathogens, against which the
principal defence is the production of
antibodies against their surface structures.
Pathogens can change their antigens so that
antibodies and memory cells are useless.
The influenza virus is typical of this, and this
is why you need a new ‘flu jag every year.
Antigenic Variation
• Antigenic variation is a change in surface antigens
on an infectious organism to help the organism
evade the immune systems of potential hosts.
• Organisms use a variety of tactics for changing the
composition of the antigens on their surface.
• This evolutionary trick allows them to continue
growing and spreading in populations, perpetuating
their existence.
• Antigenic variation is of interest for people in
charge of developing vaccines and medications to
prevent and treat infection.
Antigenic Variation
• Organisms like viruses, bacteria, and parasites all have an
external envelope, with a series of surface proteins.
• When an organism enters a host for the first time, the
immune system does not recognize any of the proteins
and may allow the organism to multiply, creating an
infection.
• The immune system will learn that those proteins are
dangerous, and when the organism appears in the future,
the body will go on the attack.
• It sees the proteins, recognizes them as a threat, and
sends out immune cells to kill the organism.
Antigenic Variation
• Without antigenic variation,
infectious organisms would
quickly become extinct.
• Numbers of vulnerable people in
the population would drop and
the organisms would not be able
to survive.
• If, however, the organism can
change the proteins in future
generations, it can adapt and
start evading the immune
system again.
Antigenic Variation
• Some organisms experience random
mutations, which can occur at any time.
• Others actually program in antigenic
variation. These organisms can switch
proteins on and off to present a
completely different antigen to the
immune system.
Antigenic Variation
• Antigenic variation can happen through mutation.
• Some organisms are better at it than others.
– The influenza viruses are a notorious example; they
change so much that people must design a new vaccine
every year to inoculate people against the flu.
– malaria – antigenic variation occurs within a population
– Trypanosomiasis - Trypanosomes are insect-borne
protozoa that replicate in the extracellular tissue
spaces of the body and cause sleeping sickness in
humans. They select from a range of genes for antigen
production
Trypanosomiasis
A protozoa called Trypanosoma brucei
causes the fatal disease called
trypanosomiasis or ‘sleeping-sickness’.
It has a glycoprotein coat which can contain
one of many different antigens. About 1%
of them can vary the antigen and replace
the whole ‘coat’ when the immune system
attacks.
The new coat has different antigens so the
immune system has to start again, giving it
time to reproduce before destroying
another 99%.
Survive and
change
Survive and
change
Malaria
To prevent the red blood cells, which the
malaria protozoa hide in, from being
destroyed they present a protein on the
cell’s surface which causes it to stick to
a blood vessel wall.
It is also able to switch between many
genes for this protein preventing the
immune system producing appropriate
antibodies in time.
Tuberculosis T.B.
Mycobacterium tuberculosis causes T.B.
It interferes with the body’s phagocytic response
The pathogen is able to survive inside phagocytes
When a macrophage engulfs the BT bacterium, the microbe prevents
lysosomes fusing with the vesicle
If fusion does occur, the pathogen is not easily attacked by lysosomal
enzymes because it is protected
The pathogen remains alive inside the phagocyte and avoids immune
detection
Tuberculosis bacteria
(green) within a
phagocyte of the immune
system
Direct attack on the immune
system
Some microorganisms hijack the immune
system for themselves, e.g. tuberculosis
bacteria
They allow themselves to be engulfed by
phagocytes but prevent the lysosomes
fusing with the vacuole they are in. They
also have a waxy coat to prevent them
being digested even if they did come into
contact with it.
This means they are able to hide amongst
the immune system and evade destruction.
Direct attack on the immune system
AIDS is a deficiency disease caused by the HIV virus.
The virus enters helper T cells by using a glycoprotein
anchor.
It changes the genetic makeup of the host cell and
inserts genes for making new virus particles.
In the case of HIV, the virus can remain dormant for
several years before releasing new virus particles.
The immune system does release antibodies against
HIV but they are ineffective against those hiding
inside the helper T cells.
After many years, the number of helper T cells has
diminished significantly and the person is now at
more risk of opportunistic infections such as
pneumonia.
AIDS (acquired immune deficiency syndrome) is a disease
caused by HIV (human immunodeficiency virus)
HIV attacks helper T lymphocytes
The envelope surrounding the HIV particle fuses with the
membrane of the helper T cells and the virus enters the
host cell
It can remain dormant for years before directing the
synthesis of new viral particles
New viral particles escape from the helper T cell by
‘budding’
B cells do make antibodies but these are ineffective
against viral particles ‘hiding’ inside helper T cells
As number of helper T cells decreases, immunological
activity decreases leaving the person susceptible to
infection
Remember, helper T cells are of critical importance to
the immune system – they activate B cells and cytotoxic T
cells
Retrovirus
HIV is a retrovirus, it contains RNA
Along with RNA it introduces ‘reverse
transcriptase’ into the host cell
This produces viral DNA from viral RNA
Eventually, viral DNA directs the synthesis of
new viral RNA
Case study on HIV-page 344.
1. Explain how HIV can be transmitted.
2. What public health measures have been put
in place to control the spread of aids?
3. Is there a cure for aids?
4. What can some drugs do?
5. What’s the problem with developing
countries?
6. What are anti-retroviral drugs?
7. What do combinations of the drugs available
do?
8. Why has a successful vaccine not been
discovered?
Clonal selection-may need.
•
•
•
These produce Y-shaped protein molecules in response to
the surface antigens of bacteria and viruses. The proteins
produced are antibodies. It is the shape of the protein of
the antibody that makes it specific to a specific antigen
Production of cell products such as antibodies in response
to an antigen is called the humeral response.
The stages in the humeral response shown by B
lymphocytes are in the diagram.
1. The B lymphocyte comes into contact with a specific
antigen from a bacteria or virus
2. The B cell divides rapidly to form a group of identical cells
3. Some cells develop into plasma cells that produce the
antibody specific to the antigen
4. The released antibody leads to the destruction of cells
with the antigen
5. Other cells develop into memory cells and if the same
antigen enters the body again, the memory cells quickly
produce more of the plasma cells.
Chapter 24 herd immunity and antigenic variation
.
question
answer
What is herd immunity?
This is protection given indirectly to the nonimmune minority by the immune majority
because a certain amount of people have been
vaccinated against a pathogen.
How does this work?
Most of a population is immunised against a pathogen
and some are not. This reduces the chances of a nonimmune individual coming into contact with an
infected person. This also gives protection to
vulnerable groups.
Name some diseases that mass
vaccination in herd immunity has
eradicated.
Tuberculosis, poliomyelitis, smallpox.
What is the herd immunity threshold?
This is the percentage of immune individuals above
which a disease can’t take hold and it changes for each
disease.
Name some factors that this threshold
depends on.
The virulence of the pathogen, the
efficacy(effectiveness) of the vaccine, the populations
contact parameters(population density).
Chapter 24-antigenic variation.
question
Answer.
Name two protozoans that can operate
antigenic variation.
Trypanosomiasis and malaria.
What disease is caused by
Trypanosomiasis?
A neurological disease called
Trypanosomiasis (sleeping sickness).
What is trypanosma brucei?
The protozoan(single celled animal) that
causes this disease.
What is the pathogen that causes this
surrounded by that can vary in chemical
composition depending on which genes
are switched on?
A coat of glycoprotein molecules.
What is made by the infected host in
response to the antigen (the pathogen’s
coat) and kills 99% of the protozoa?
Antibodies.
What happens to the other 1%?
Get rid of their coat, switch on other genes and
get a new antigenic coat so the hosts immune
system makes a new set of antibodies that deals
with 99% again and 1% shed their coat and
survive etc. until host dies.
Chapter24 antigenic variation-malaria.
Question.
Answer.
What pathogen causes malaria?
A protozoan called plasmodium falciparum.
Describe its life cycle.
Complex .Found in humans and mosquitoes
and humans and mosquitoes are called the
hosts. Found for longer in humans in their red
blood cells.
What allows the pathogen to evade the hosts
immune system?
Great genetic variation exists in the pathogen
that it switches between.
It makes a protein that sticks to Red blood
cells surface and stops it from being moved or
destroyed by the body to allow the pathogens
to survive in the RBC.
Have scientists been able to produce an
effective vaccine against it?
No and malaria kills many people every year.
Chapter 24 direct attack on the immune system
question
Answer.
From which cells are phagocytes derived?
Bone marrow stem cells.
Do they exert a specific or non-specific
attack on pathogens?
Non-specific.
How do they destroy pathogens?
Engulf and digest using enzymes in
lysosomes.
What are larger phagocytic cells called?
Macrophages and are found in connective
tissue.
Where are the smaller phagocytes found?
In the blood stream.
What can some bacteria do to succeed?
They can interfere with the phagocytic
response and stop it.
Name the bacteria that causes
tuberculosis.
Mycobacterium tuberculosis.
Why is it called an intracellular pathogen?
It can survive inside phagocytes.
How can it do this?
Once inside a vesicle in the macrophage it stops lysosomes
fusing with the vesicle and if fusion does happen it is protected
by a waxy cell wall and can’t be digested-other macrophages
may surround it.
Chapter 24 immunodeficiency disease.
Question.
Answer.
Define immunodeficiency disease.
Some part of the immune system does not
work and the person is open to infection.
What does AIDS stand for?
Acquired immune deficiency syndrome.
What does HIV stand for?
Human immunodeficiency virus.
How does HIV work?
The virus attacks helper T cells by
becoming attached by glycoprotein on its
surface to specific receptors on the helper
T cells. The HIV particle fuses with the
membrane of the helper T cell and the
virus enters the cell.
Why is HIV described as a retrovirus?
Because it contains RNA and not DNA.
How does the virus make viral DNA?
It uses the enzyme reverse transcriptase to
reverse normal transcription and makes viral
DNA from viral RNA. This can stay in the host
cell’s DNA for many years before making new
viral particles inside the host cell and then
leaving by budding to infect other cells. The host
cell gets destroyed by apoptosis.
HIV continued.
question
answer
Why are the antibodies against HIV
made by B cells not effective against
viral particles?
Viral particles are inside the helper T
cells and antibodies can’t get to them.
Why are helper T cells important for the They activate B cells and cytotoxic T
immune system?
cells .
What happens as the number of helper
T cells drop?
The immune system does not work as
well and the person is susceptible to
opportunistic infections such as
pneumonia. The person develops aids
after several years of infection.
Chapter 24 herd immunity and antigenic variation
.
Question
Answer.
What is the public health policy for
combating common diseases in many
countries?
They carry out mass vaccination
programmes that create herd immunity to
the pathogens.
What is this policy in the UK?
Vaccination for diphtheria, tetanus , polio,
whooping cough, flu begins at 2 months
and continues for many years.
Where may herd immunity be absent?
Developing countries where many people
are poor and malnourished, developed
countries where vaccination preventable
diseases are compromised due to people
believing bad publicity about the vaccine.
i.e. the level of herd immunity is below
threshold and non-vaccinated people do
not get protection.
Chapter 24-evasion of specific immune responses
.
question
answer
How can a pathogen evade specific
immune response?
They do this by evolving into new versions
of the pathogen to stay one step ahead of
the vaccination programme.
How can pathogens change their
genotype?
By mutations and genetic recombination
which is a combination of genetic material
from two different strains.
What is antigenic variation?
This is where pathogens have antigens on
their surface different from the original
strain.
How does the influenza virus avoid the
body’s immune system?
It produces new antigens which re-infect
the body because its new antigens are not
recognised by the body’s memory cells. At
risk individuals are vaccinated every year
with a new version of the vaccine.
Apart from viruses and bacteria what
other pathogens can operate antigenic
variation?
Pathogenic protozoa (unicellular animals).