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GCSE Biology Revision Notes
B.5.Immune System
THE IMMUNE SYSTEM
Microorganisms that cause disease include fungi, bacteria, protozoa and viruses and are
collectively known as pathogens (adjective = pathogenic). Microbes most often enter the body
through respiratory surfaces (lungs), breaks in the skin, the digestive system, or the urethra.
Non-specific Defences
These consist of physical barriers that block entry of microorganisms, such as:

Skin – a barrier that cannot be penetrated without abrasions.

Saliva and tears – these contain antimicrobial enzymes that digest the cell walls of many
kinds of bacteria.

Mucous membranes which line the openings of an animal’s body and secrete mucus e.g.
mucus in the lining the respiratory tract which trap microbes, which are then swept
away by the ciliated epithelial cells to prevent them form entering the lungs (see notes
on breathing).

Acidic environment of the stomach kills many bacteria that have been ingested with
food.
Cells of the Immune System
When microorganisms penetrate the first line of defence, they may encounter white blood cells
whose function is to kill and remove pathogens from the body.
Phagocytes
Phagocytes are attracted to sites of infection by chemical signals produced by cells damaged by
the microorganism. They engulf the pathogen by the process of phagocytosis (see diagram
below).
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GCSE Biology Revision Notes
B.5.Immune System
Lymphocytes
In contrast to phagocytes, lymphocytes recognize and respond to particular pathogens and
foreign molecules, so their response is said to be specific.
Any foreign molecule that elicits this specific response is called an antigen. An antigen could be
a molecule that makes up part of the cell wall of a bacterial cell, of perhaps a protein on the
outside of a virus. What is important is that the lymphocyte can recognize it as a foreign
molecule i.e. one that would not normally be found in the body.
Each antigen has a particular molecular shape, which will activate certain lymphocytes to
secrete proteins called antibodies.
Lymphocytes have receptors that recognise and bind to different antigens. This is similar to
how an enzyme will bind only to a specific substrate because the substrate fits in the active site
of the enzyme. The lymphocyte’s receptors don’t have active sites (we only use that term for
enzymes), but they do have binding sites that the antigen will bind to.
There is an enormous variety of lymphocytes in the body, each bearing a receptor that will
recognise a specific antigen. This means the immune system has the capacity to respond to
millions of different pathogens.
When a lymphocyte encounters its correct antigen, it becomes activated and starts to (a) divide
by mitosis and (b) the cells start to secrete antibodies. Antibodies are similar to the receptors on
the lymphocyte that initially encountered the antigen in the first place.
Antibodies have a Y shaped structure, and they are produced in large quantities by the
lymphocytes. Because they are specific to the antigen that caused their production, they can
bind to the antigens in the following ways:

Neutralisation – the antigens bind to and block activity of antigen. e.g. by binding to
antigens on the outside of a virus they stop it being able to infect host cell.

Coat pathogen to make it more obvious to phagocytes, so they can engulf it by
phagocytosis as decribed above.

Clump the microbes together (agglutination) to neutralise their effects and make them
easier targets for phagocytes (see diagram below).
Antibodies bound to cells, causing them to clump together.
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GCSE Biology Revision Notes
B.5.Immune System
Immunological Memory (triple award only)
Lymphocytes also allow for long term protection from antigens they have previously been
exposed to. This is called immunological memory, and is the basis of vaccinations against
diseases.
The first time the body is exposed to an antigen the lymphocytes are activated, they divide and
start to produce antibodies. This is called the primary immune response. This process all takes
time, around 10-17 days from initial exposure to the antigen for the maximum production of
antibodies. During this time, the infected person may experience symptoms of illness before the
antigen is cleared from the body.
However, some of the lymphocytes will persist in the body after the initial infection. They are
called memory cells. These are lymphocytes that are primed for action next time the body
encounters the same antigen for a second time.
When memory cells encounter the same antigen at a later time, the response is faster (2-7days),
of greater magnitude, and more prolonged. This is called the secondary immune response.
Measures of antibody concentrations in the blood over time show clearly the difference between
primary and secondary immune responses, see the graph below:
The body was exposed to two different antigens – “antigen A” and “antigen B”. Antigen A was
injected first, and after a few days the amount of antibody against antigen A starts to rise before
falling back to a lower level. This is the primary immune response.
The, on day 7, more of antigen A was injected. This time the production of antibodies by
lymphocytes was quicker and more were produced. This is the secondary response. By
producing so many antibodies against the antigen so rapidly it means that the body is less likely
to suffer from the disease compared with the first time that the body encountered it.
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GCSE Biology Revision Notes
B.5.Immune System
Also on day 7, a different antigen, antigen B, was injected. Because the body hadn’t encountered
this particular kind of antigen before it had to wait until a lymphocyte with the correct receptor
for the B antigen was activated and divided before antibody was produced. In other words, the
body had to go through the primary immune response again, this time for antigen B.
Vaccinations (Triple award only)
The ability of the immune system to ‘remember’ antigens that it has previously encountered is
the basis for vaccinations – an artificial way of producing long lasting immunity to particular
pathogens.
Vaccines can be made up of:

inactivated toxins (chemicals produced by pathogens)

killed microbes

parts of microbes

viable but weakened microbes.
They can no longer cause disease, but they can retain the ability to act as antigens and stimulate
an immune response which leads to the production of memory cells.
As such, when a vaccinated person encounters that particular pathogen for real, they can elicit a
quick secondary immune response without suffering the symptoms of the disease.
N.B. not all disease can be vaccinated against due to antigenic variability, e.g. HIV.
Blood Clotting (Triple award only)
Platelets release clotting factors which start a chain of complex reactions which result in
fibrinogen, a soluble protein, being converted, fibrin, an insoluble form.
Fibrin aggregates into threads that form the fabric of a clot. The network of fibrin traps red
blood cells and the mesh eventually dries out, forming a barrier, the clot, which prevent further
blood loss and seals the area against further infection until new tissue grows and heals the
wound.
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