Cambridge (CIE) IGCSE Your notes
Biology
Diseases & Immunity
Contents
Pathogens & Barriers
Controlling the Spread of Disease
Active Immunity
Antigens & Antibodies
Vaccination
Preventing the Spread of Disease
Passive Immunity & Breastfeeding
Cholera
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Pathogens & Barriers
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Pathogens
A pathogen is a disease-causing organism
Pathogens are passed on from one host to another and therefore the diseases they
cause are known as transmissible diseases
Pathogens can be passed on from host to host in different ways, including:
Direct contact - the pathogen is passed directly from one host to another by
transfer of body fluids such as blood or semen (eg HIV, gonorrhoea, hepatitis B & C)
Indirect contact - the pathogen leaves the host and is carried by an intermediate
medium or vector, such as contaminated surfaces, air (via droplets or aerosols),
food, water, or organisms like insects (e.g., mosquitoes carrying malaria or dengue)
Methods of Transmission Table
The Body Defences
There are 3 main ways in which the body defends itself against disease:
1. Mechanical barriers – structures that make it difficult for pathogens to get past them
and into the body
1. Skin - covers almost all parts of your body to prevent infection from pathogens. If it
is cut or grazed, it immediately begins to heal itself, often by forming a scab
2. Hairs in the nose - these make it difficult for pathogens to get past them further up
the nose so they are not inhaled into the lungs
2. Chemical barriers – substances produced by the body cells that trap / kill pathogens
before they can get further into the body and cause disease
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1. Mucus - made in various places in the body, pathogens get trapped in the mucus
and can then be removed from the body (by coughing, blowing the nose,
swallowing etc) Your notes
2. Stomach acid - contains hydrochloric acid which is strong enough to kill any
pathogens that have been caught in mucus in the airways and then swallowed or
have been consumed in food or water
3. Cells - different types of white blood cell work to prevent pathogens reaching areas of
the body they can replicate in
1. By phagocytosis - engulfing and digesting pathogenic cells
2. By producing antibodies - which clump pathogenic cells together so they can’t
move as easily (known as agglutination) and releasing chemicals that signal to other
cells that they must be destroyed
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Controlling the Spread of Disease
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Controlling the Spread of Disease
The simplest way to prevent disease is to stop pathogens from spreading
This means using simple measures such as good hygiene and effective sanitation and
waste disposal to contain pathogens and dispose of them safely
Preventing the Spread of Disease Table
Measure to Importance
prevent spread
Clean water Safe drinking water ensures water is free of pathogens which will
supply prevent waterborne diseases e.g. cholera.
Hygienic food Washing hands with soap, correct food cooking methods and
preparation hygienic preparation of food will help to prevent contamination of
food with harmful bacteria and fungi,
Personal hygiene Washing with soap or using tissues to catch sneezes and coughs and
then disposing of them will reduce transmission of diseases such as
colds and flu.
Waste disposal Proper disposal of waste e.g. food waste will reduce the prevalence
of pests e.g. flies that can act as vectors for transmissible diseases.
Sewage Treatment of sewage removes harmful pathogens from waste before
treatment it is released into the environment. This reduces the risk of diseases
spreading in contaminated water.
FlashcardsNext topic
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Active Immunity
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Active Immunity: Extended
Extended Tier Only
Making antibodies and developing memory cells for future response to infection is
known as active immunity
There are two ways in which this active immune response happens:
The body has become infected with a pathogen and so the lymphocytes go
through the process of making antibodies specific to that pathogen
Vaccination
Active immunity is slow acting and provides long-lasting immunity
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Antigens & Antibodies
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Antigens & Antibodies: Extended
Extended Tier Only
All cells have molecules, such as proteins, projecting from their cell membranes
These are known as antigens
Different individuals have different antigens on their cell surface membranes
Lymphocytes can recognise foreign antigens, e.g. the antigens of a pathogen inside the
body
In response to foreign antigens lymphocytes make antibodies which are
complementary in shape to the antigens on the surface of the pathogenic cell
Lymphocytes producing antibodies diagram
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Your notes
Lymphocytes produce antibodies that are specific to a particular type of antigen
The role of antibodies
Antibodies can attach to antigens and cause agglutination of pathogens
This means the pathogenic cells cannot move around very easily
At the same time chemicals are released that signal the presence of pathogens to
phagocytes
Phagocytes move towards the site of an infection where they engulf and destroy
pathogens
Many pathogens can be engulfed together due to agglutination
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Phagocytes and agglutination diagram
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Agglutinated pathogens cannot move around so phagocytosis can occur more easily
Immunity
The initial response of a lymphocyte encountering a pathogen for the first time can take
a few days, during which time an individual may get sick
After an initial encounter with a pathogen, lymphocytes can give rise to memory cells
that retain the instructions for making specific antibodies
This means that in the case of reinfection by the same type of pathogen, antibodies can
be made very quickly and in greater quantities; the pathogens are destroyed before
they multiply and cause illness
This is how people become immune to certain diseases after only having them once
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Note that this does not work with all disease-causing microorganisms as some of them
mutate quickly and change the antigens on their cell surface
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Therefore, if they invade the body for a second time the memory cells made in the
first infection will not produce antibodies that match the new antigens
Examiner Tips and Tricks
Make sure you know the difference between antigen, antibody and antitoxin:
An antigen is a molecule found on the surface of a cell
An antibody is a protein made by lymphocytes that is complementary to an
antigen and, when attached, clumps them together and signals the cells they are
on for destruction
An antitoxin is a protein that neutralises the toxins produced by bacteria
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Vaccination
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Vaccination: Extended
Extended Tier Only
Vaccinations give protection against specific diseases and boost the body’s defence
against infection from pathogens without the need to be exposed to dangerous
diseases that can lead to death
The level of protection in a population depends on the proportion of people vaccinated
Vaccines allow a dead or altered form of the disease-causing pathogen, which contains
specific antigens, to be introduced into the body
In this weakened state, the pathogen cannot cause illness but can provoke an immune
response
Lymphocytes produce complementary antibodies for the antigens
The antibodies target the antigen and attach themselves to it in order to create memory
cells
The memory cells remain in the blood and will quickly respond to the antigen if it is
encountered again in an infection by a ‘live’ pathogen
As memory cells have been produced, this immunity is long-lasting
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Vaccination
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Preventing the Spread of Disease
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Preventing the Spread of Disease: Extended
Extended Tier Only
If a large enough percentage of the population is vaccinated, it provides protection for
the entire population because there are very few places for the pathogen to breed - it
can only do so if it enters the body of an unvaccinated person
This is known as herd immunity
If the number of people vaccinated against a specific disease drops in a population, it
leaves the rest of the population at risk of mass infection, as they are more likely to
come across people who are infected and contagious This increases the number of
infections, as well as the number of people who could die from a specific infectious
disease
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Your notes
Herd immunity
Herd immunity prevents epidemics and pandemics from occurring in populations
This is the reason that many vaccinations are given to children, as they are regularly seen
by medical practitioners and can be vaccinated early to ensure the entire vaccinated
population remains at a high level
In certain instances, vaccination programmes are run with the aim of eradicating certain
dangerous diseases, as opposed to controlling them at low levels
An example of a disease which has been eradicated as a result of a successful
vaccination programme is smallpox, which was officially eradicated in 1980 after a
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vaccination programme run by the World Health Organisation since the mid-1950s
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Passive Immunity & Breastfeeding
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Passive Immunity & Breastfeeding
Passive immunity is a fast-acting, short-term defence against a pathogen by
antibodies acquired from another individual
Antibodies pass from mother to infant via breast milk - this is important as it helps the
very young to fight off infections until they are older and stronger and their immune
system is more responsive
The body does not make its own antibodies or memory cells in passive immunity, hence
the name
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Cholera
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Cholera
Cholera causes diarrhoea
Diarrhoea is the loss of watery faeces from the anus
If it is severe and continues for a long time, it can lead to death
Severe diarrhoea can cause the loss of significant amounts of water and ions from the
body, causing the tissues and organs to stop working properly
It can be effectively treated by oral rehydration therapy
This is a drink with a small amount of salt and sugar dissolved in it
There are many causes of diarrhoea, one of which is infection with Vibrio cholerae
bacteria, which causes the disease cholera
How does Vibrio cholerae cause diarrhoea?
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Your notes
How cholera leads to diarrhoea
Ingested via infected water or food, if it enters the small intestine it can cause illness in
the following way:
1. Bacteria attach to the wall of the small intestine
2. They produce a toxin
3. The toxin stimulates the cells lining the intestine to release chloride ions from inside the
cells into the lumen of the intestine
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4. The chloride ions accumulate in the lumen of the small intestine and lower the water
potential there
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5. Once the water potential is lower than that of the cells lining the intestine, water starts to
move out of the cells into the intestine (by osmosis)
6. Large quantities of water are lost from the body in watery faeces
7. The blood contains too little chloride ions and water
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