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O Level Biology Cambridge Your notes
12.3 Immunity
Contents
Active Immunity
Antigens & Antibodies
Vaccination
Preventing the Spread of Disease
Passive Immunity & Breastfeeding
HIV & the Immune System
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Active Immunity
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Active Immunity
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
All cells have proteins and other substances projecting (sticking out) from their cell membrane
These are known as antigens and are specific to that type of cell
Lymphocytes have the ability to ‘read’ the antigens on the surfaces of cells and recognise any
that are foreign
They then make antibodies which are a complementary shape to the antigens on the surface of
the pathogenic cell
Antibodies can bind to antigens because of their complementary shapes
Like jigsaw puz z le pieces fitting together
Antigens and Antibodies Diagram
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Antigens and antibodies
The antibodies attach to the antigens and cause agglutination (clumping together) Your notes
This means the pathogenic cells cannot move very easily
At the same time, chemicals are released that signal to phagocytes that there are cells present
that need to be destroyed
Agglutination of Pathogens Diagram
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Agglutinated pathogens cannot move easily
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The initial response of a lymphocyte encountering a pathogen for the first time and making
specific antibodies for its antigens can take a few days, during which time an individual may
become ill Your notes
Lymphocytes that have made antibodies for a specific pathogen for the first time will then make
‘memory cells’ that retain the instructions for making those specific antibodies for that type of
pathogen
This means that in the case of reinfection by the same type of pathogen, antibodies can be
made very quickly, in greater quantities
The pathogens get destroyed before they are able to multiply and cause illness
This is how people can become immune to certain diseases after only having them once
It does not work with all disease-causing microorganisms as some of them mutate fairly quickly
and change the antigens on their cell surfaces
Therefore, if they invade the body for a second time, the memory cells made in the first infection
will not recall them as they now have slightly different antigens on their surfaces (e.g. the cold
virus)
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Vaccination
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Vaccination
Vaccinations give protection against specific diseases and boost the host’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 weakened form of the disease-causing pathogen, which contains specific
antigens, to be introduced into the body
In this weakened state, the pathogen is harmless and cannot cause illness
But it can provoke an immune response
There are sometimes mild symptoms of the disease after vaccination, but not the full-scale
disease
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
Vaccination Process Diagram
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Vaccination
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Preventing the Spread of Disease
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Preventing the Spread of Disease
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
Herd Immunity Diagram
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Herd immunity Your notes
Herd immunity prevents epidemics and pandemics from occurring in populations
This is the reason that many vaccinations are given to children
Children are seen regularly 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 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
Before birth, antibodies can be transmitted across the placenta from other to fetus
After the birth of an infant, antibodies pass from mother to infant via breast milk which contains
some white blood cells that can themselves produce antibodies
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 baby does not make its own antibodies or memory cells in passive immunity, hence the
name
The lack of memory cells explains why passive immunity is temporary
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HIV & the Immune System
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HIV & the Immune System
The HIV virus infects a certain type of lymphocyte in the immune system
Normally lymphocytes seek out and destroy pathogens that enter the body, producing
antibodies that attach to pathogens, enhancing phagocytic activity
However, HIV infects a certain type of lymphocyte and uses the cells’ machinery to multiply
This reduces the number of lymphocytes in the immune system, and also the number of
antibodies that can be made
This decreases the body’s ability to fight off infections, eventually leading to AIDS (Acquired
immunodeficiency Syndrome)
The weakened immune system is susceptible to infections that would normally have been dealt
with by a fully-functioning immune system
e.g influenz a, pneumonia, skin cancer
Exam Tip
This page focuses on HIV and the immune system, but for more detail on the HIV virus and its
effects, see topic 12.1 Human Immunodeficiency Virus
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