KEEPING HEALTHY - The bodies defence against infection
Doc Brown's Biology Revision Notes
Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent
This page will answer many questions e.g.
How does our body defend itself when it
What are the physical and chemical methods of
What is a pathogen?
What is our immune system?
What is a vaccine?
How does vaccination protect us?
How our bodies defend themselves against infectious diseases?
A simple example of how science
works - cleanliness reduces the incidence of infection!
Be able to relate the contribution of
Semmelweiss in controlling
the rate of patient infection to solving modern problems with the spread
of infection in hospitals.
Semmelweis worked in Vienna
General Hospital in the 1840s and witnessed large numbers of women dying
after childbirth from a puerperal fever disease.
He thought that the staff of the
hospital were spreading the disease via unwashed hands.
After instructing doctors and
nurses to wash their hands in an antiseptic solution, the mortality rate was
Although Semmelweis didn't
realise it at the time, the antiseptic solution was killing the infecting
Apparently, when he left the
Vienna hospital, the practice of washing hands in the antiseptic solution
was relaxed, and the death rates rose again!
With the advent of new strain of
bacteria today, there is now an even greater need for emphasis on hospital hygiene
than ever before - so, if on a hospital visit, PLEASE WASH YOUR HANDS in the
antiseptic gel provided.
Be able to explain how the treatment of disease has changed
as a result of increased understanding of the action
of antibiotics and immunity.
Be able to evaluate the consequences of mutations of bacteria
and viruses in relation to epidemics and pandemics
- data provided.
Be able to evaluate the advantages and disadvantages of
being vaccinated against a particular disease - data provided.
Know that ...
a) Microorganisms that cause infectious disease are
b) Bacteria and viruses may reproduce rapidly inside
the body and may produce poisons (toxins) that
make us feel ill.
What is a bacteria? What is a
pathogen? What is a virus?
Bacteria and certain protozoa are very
small cells which can rapidly reproduce by cell division in your body making you feel ill by
damaging your body's cells and producing toxins (poisons produced as a
by-product of the bacteria's cell chemistry).
Viruses are NOT
cells and much smaller than bacteria and damage the cells in which
Viruses replicate by invading a
cell and using the cell's genetic machinery to reproduce themselves ie
copies of the original virus.
The virus 'invaded' cell then
bursts releasing lots of new viruses.
The cell damage makes you feel
ill as your body (temporarily) fights back to make as many good cells as it
can to replace those destroyed by the virus.
(Knowledge of the structure of bacteria and viruses is
not required here.)
Fungi are also pathogens
and includes microorganisms like yeasts and moulds (so don't eat mouldy
c) The body has different
physical and chemical ways of protecting itself
d) White blood cells help to defend against pathogens by:
Producing antibodies, which destroy particular
bacteria or viruses.
What is the function of white
What is an antibody?
antibodies? What is an antigen? What is an antitoxin?
All invading cells have unique
molecules ('molecular structure') on their surface called antigens.
When white cells encounter a
'foreign' antigen on a pathogen they don't recognise, they produce proteins
called antibodies which lock onto the antigens of the pathogen.
The antibodies produced are
specific to that type of antigen, they will not lock onto any other type of
antigen, hence they are specific to a particular pathogen.
One the 'blueprint' antibody is
made, it is rapidly reproduced and they lock onto the invasive microbes and
kill these bacteria.
If a person becomes infected
with the same microbic pathogen, the white blood cells will automatically,
and quickly, produce the antibodies to kill the pathogen because of the
first invasion of a particularly pathogen the person has become immune.
This immunity helps prevent the immune person becoming ill again, or at
least minimises the 2nd attack of the specific pathogen.
Producing antitoxins, which counteract the toxins
released by the pathogens.
White blood cells are one of the
most important parts of the body's defences known as the immune system.
What is the immune system?
The immune system 'kicks in' if
pathogens do get inside your body.
The white blood cells are
present throughout your body in your blood system and therefore are always
at hand to defend you from invading pathogens.
If your white blood cell count
is low you are more susceptible to disease and infection.
For example, HIV/AIDS weakens
white cell action and hence the body has a weaker responding immune system
that allows pathogens to have a more devastating effect on the body -
sometimes with fatal consequences from a disease that in a healthy body
would not have proved fatal.
e) The immune system of the body produces specific
antibodies to kill a particular pathogen.
This leads to
immunity from that pathogen. In some cases, dead
or inactivated pathogens stimulate antibody
If a large proportion of the population
is immune to a pathogen, the spread of the
pathogen is very much reduced.
f) Semmelweiss recognised the importance of
hand-washing in the prevention of
spreading some infectious diseases.
g) Some medicines, including painkillers, help to relieve
the symptoms of infectious disease, but do not kill
h) Antibiotics, including penicillin, are medicines that
help to cure bacterial disease by killing infectious
bacteria inside the body.
What is an antibiotic?
Antibiotics cannot be used
to kill viral pathogens, which live and reproduce inside
penicillin kill or prevent the growth of harmful pathogens, they kill the
bacteria but not your own body cells.
Different antibiotics attack
different bacteria, so it is important that specific bacteria should be
treated by specific antibiotics.
The use of antibiotics
has greatly reduced deaths from infectious bacterial
However, overuse and inappropriate use of antibiotics
has increased the rate of development of antibiotic
resistant strains of bacteria.
You need to be aware that it is difficult to develop
drugs that kill viruses without also damaging the body’s
- Explaining the use of antibiotics to
- Antibiotics are taken internally e.g.
intravenous syringe injection, or orally taken tablet or liquid suspension.
- Antibacterials to treat bacterial infections
- Probably the most well known antibacterial
is the antibiotic penicillin which is effective against many bacterial
infections BUT NOT viruses like the common cold or flue.
- An antibiotic can kill bacteria or prevent
them growing and reproducing.
i) Many strains of bacteria, including MRSA, have
developed resistance to antibiotics due to mutations, which cause stronger more
resilient strains of bacteria to survive as a result of
To prevent further resistance
arising it is important to avoid over-use of antibiotics.
Knowledge of the development of resistance in bacteria
is limited to the fact that pathogens mutate, producing
Mutations of pathogens produce new strains.
Antibiotics and vaccinations may no longer be effective
against a new resistant strain of the pathogen.
strain will then spread rapidly because people are not
immune to it and there is no effective treatment.
Can bacteria become resistant
Unfortunately the answer is yes!
Bacteria will sometimes quite naturally mutate into forms that are resistant
to current antibiotics, so if your infected with a new strain of bacteria,
your resistance is not as effective.
If an infection is treated with
an antibiotic, any resistant bacteria will survive and this means resistant bacteria
can survive and reproduce to infect other people, while the non-resistant
strains will tend to be reduced.
This is an example of natural
selection at the individual cell level and drug companies are constantly
trying to develop new antibiotics to combat the new evolving strains of
harmful bacteria - but new harmful 'superbugs' are becoming more common the
more we use antibiotics and new epidemics can break out!
staphylococcus aureus, can't be treated with many current antibiotics and
causes serious wound infections that can be fatal to young babies or elderly
people in particular.
Misuse by over-prescribing antibiotics is
believed to be causing the rise of mutant resistant strains of bacteria, so
doctors are being advised to avoid over-prescribing antibiotics to reduce
the mutation rate and not treating mild infections with antibiotics.
It isn't just bacteria that can
mutate, viruses can also evolve via new mutations. Viruses are
notable for the rapidity with which they can mutate which makes it difficult
to develop new vaccines. The reason being that changes in the virus (or
bacteria) DNA leads to different gene expression in the form of different
antigens, so different antibodies are needed. The flue virus is a never
ending problem and in the past pandemics (epidemics across many countries at
the same time) have killed millions of people, mercifully this rarely
happens these days thanks to antibiotics.
HT only: Understand that antibiotics kill individual pathogens of the
Individual resistant pathogens survive and
reproduce, so the population of the resistant
Now, antibiotics are not used to treat
non-serious infections, such as mild throat
infections, so that the rate of development of
resistant strains is slowed down.
k) The development of antibiotic-resistant strains of
bacteria necessitates the development of new antibiotics.
l) People can be immunised against a disease by
introducing small quantities of dead or inactive forms
of the pathogen into the body (vaccination).
Know that vaccination is an
important method of
What is vaccination? What is a vaccine?
Vaccination protects the
individual from future infections and mass scale vaccination can greatly
reduce the incidence of disease.
Protection is better than cure!
If you become infected with a pathogen, it takes a few days for your white
cell immune system to deal with the microorganism, and you can become quite
ill in a few days.
Vaccination is the
process of injecting the individual with small amounts of specific harmless
dead/inactive microorganisms which carry the antigens that cause the immune
system to produce the corresponding protective antibodies.
stimulate the white blood cells to produce antibodies
that destroy the invading 'foreign' pathogens.
This makes the person
immune to future infections by the microorganism ie gives the individual
immunity from further attacks.
The body can respond by rapidly making the correct
antibody, in the same way as if the person had
previously had the disease.
MMR vaccine is used to
triple protect children against
measles, mumps and rubella (German measles).
The effects of vaccination can
'wear off' over time, and booster injections maybe necessary to increase the
levels of the protective antibodies.
There are arguments for and
against vaccination (the 'pros and cons').
For: Vaccines have
resulted in the large scale control of many infectious diseases that were
once common and often fatal eg measles, mumps, polio, rubella, smallpox,
tetanus, whooping cough etc. Epidemics are less likely with mass vaccination
- spread of the disease is less likely as there are fewer infected people
carry an active form of the disease. Without mass vaccination an outbreak of
epidemic proportions is much more likely - more people potentially to carry
Against: Some vaccines do
not always give you immunity but development work goes on all the time to
make more effective vaccines - especially as different strains of viruses
and bacteria are constantly evolving. There may also be side-effects
in which the 'patient' has a bad reaction to a particular vaccine eg
swelling, fever, seizure (serious!), but such reactions and complications
are rare and the mass good effect is balanced against the very rare negative
There is a very small risk
involved with most medical treatments and although side-effects are not
uncommon, without vaccination some of these diseases are fatal or have very
serious non-fatal outcomes - people can die of from measles, rubella has
serious consequences for pregnant women, there can be serious complications
for infected people who have not been vaccinated.
Following a seaside accident, as
an eleven year old, I collapsed unconscious after a tetanus injection at a
local hospital. I was ok within half an hour BUT my parents got a bit of a
Details of vaccination schedules and side effects
associated with specific vaccines are not required.
m) Uncontaminated cultures of microorganisms are
required for investigating the action of disinfectants
Petri dishes and culture media must be sterilised
before use to kill unwanted microorganisms.
Inoculating loops used to transfer microorganisms
to the media must be sterilised by passing them
through a flame.
The lid of the Petri dish should be secured with
adhesive tape to prevent microorganisms from
the air contaminating the culture.
n) In school and college laboratories, cultures should
be incubated at a maximum temperature of 25 °C,
which greatly reduces the likelihood of growth of
pathogens that might be harmful to humans.
o) In industrial conditions higher temperatures can
produce more rapid growth of unwanted, potentially harmful microorganisms.
Any practical work and
investigations you did should also be revised - good context material for
exam questions! See below!
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