A Search For Better Health
Topic 8: Second Line of Defence
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT Point(s)
 identify antigens as molecules that trigger the immune
response
 explain why organ transplants should trigger an immune
response
 identify defence adaptations, including:
 inflammation response
 phagocytosis
 lymph system
 cell death to seal off pathogen
Antigens
An antigen is any molecule the
body recognises as foreign and
that triggers the immune
response.
On the surface of cells in the
body, there are ‘marker’
molecules that identify the cell
as belonging to the body
(‘self’). This protects the cells in
the body from attack by its own
immune system.
www.dsto.defence.gov.au
Antigens
When pathogens enter the body
they have chemical markers,
called antigens, on their surface;
the immune system recognises
these as not belonging to the
body (‘non-self’).
 The presence of these
antigens causes the immune
response to be activated to
destroy the foreign
organisms.
cultivatingsunshine.wordpress.com
Antigens
It is not only pathogens that have
antigens on their surface.
 Any foreign cell, cell
fragment, protein debris or
toxin produced by bacteria
can also contain antigens.
The venom of poisonous snakes
contains a number of antigens.
In all these cases the immune
response will be activated
because the body recognises all
these antigens as foreign
molecules (non-self).
www.avru.org
Immune Response: Organ Transplant
When a person has an organ
transplant, the new organ has
antigens that are different to the
antigens on their own cells.
 The transplanted organ is
therefore identified as foreign
and the immune response is
activated to attack the organ in
order to defend the body.
brucemhood.wordpress.com
Immune Response: Organ Transplant
To try to prevent this from
happening, the tissue type of the
donor is matched to the
recipient as closely as possible
so that there is a high number of
matching ‘marker’ molecules.
This will mean that there are
fewer foreign (antigen)
molecules on the surface, a
situation which may lead to a
less ‘violent’ immune response.
www.fiveaa.com.au
Immune Response: Organ Transplant
The patient can also be treated
with immunosuppressant drugs,
which will also lessen the
immune response so that the
transplanted organ is not
attacked. This has the
disadvantage of making the
patient more susceptible to
infection from pathogens and
they must take precautions
(such as isolation) to reduce
their potential exposure to any
infections.
healthland.time.com
Second Line of Defence
When pathogens are successful in penetrating the barriers that are
in place to prevent their entry into the organism (the first line of
defence), non-specific responses that are the ‘second line of
defence’ are quickly activated to try to destroy the invaders before
they can cause any damage to the body.
www.biologyexams4u.com
Second Line of Defence
These non-specific defence adaptations include:
 the inflammation response
 phagocytosis
 the lymph system
 cell death to seal off the pathogens.
These processes work together, ahead of, and with the third line
of defence, if required, in order to defend the body against attack.
Students to complete the worksheet instead of taking notes on
the following section.
Inflammation Response
 The inflammation response is a non-specific defence
mechanism and occurs at the site of infection. When the cells
are infected or injured in some way, they release chemical alarm
signals such as histamines and prostaglandins.
www.biologyexams4u.com
Inflammation Response
These chemicals cause the blood vessels to dilate, increasing the
blood flow to the site of infection or injury and causing the area to
become red, hot and swollen.
en.wikipedia.org
Inflammation Response
These chemicals also increase
the permeability of the blood
vessels; this allows the
movement of phagocytes from
the blood into the tissues so they
can attack the invading
pathogens. Phagocytes are a
special type of white blood cell
and are described in more detail
in the next section.
drugline.org
Inflammation Response
Plasma also moves into the
tissues, bringing more
phagocytes and producing
swelling in the area of the
infection, forcing tissue fluid
into the lymph and taking debris
and pathogens with it.
www.dsto.defence.gov.au
Inflammation Response
Chemicals that increase the
temperature are released. This
inhibits the growth rate of
pathogens, inactivates some
enzymes and toxins made by the
pathogens and increases the rate
at which the biochemical
reactions occur in the body.
When the pathogens are
destroyed, they are removed
along with any toxins and the
tissues are repaired.
www.clinicaladvisor.com
Phagocytosis
Phagocytosis is the process by
which phagocytes change their
shape so that they can surround
a foreign particle, such as a
bacterium, and completely
enclose it within their cell.
Once it is inside the cell,
enzymes are released to destroy
the foreign material.
www.dsto.defence.gov.au
Phagocytosis
Phagocytes are specialised white
blood cells or leucocytes. The two
main types of phagocytes are:
 Neutrophils
 Macrophages.
www.bbc.co.uk
Phagocytosis
Neutrophils are the first to be
called upon and move to the site
of the infection, inactivating
pathogens. Neutrophils are
short-acting and then selfdestruct after a few days. They
are used by the body to fight
acute (short, severe) infections.
www.dsto.defence.gov.au
Phagocytosis
Macrophages are long-lasting
phagocytes that can either stay in
the tissues or travel from the
blood vessels into the infected
tissues. They are used by the
body to fight chronic (longlasting) infections.
srxawordonhealth.com
Phagocytosis
After the macrophage has destroyed
the foreign particle, parts of the
antigen are displayed on the surface
of the macrophage. We will see why
this is important later when we
learn about the bodies third line of
defence.
 Phagocytosis is not always
successful as the pathogens can
sometimes repel the phagocytes
and may escape before being
completely destroyed.
medical-dictionary.thefreedictionary.com
Lymph System
As the blood circulates around
the body, some of the plasma
moves out of the capillaries into
the tissues and becomes part of
the tissue fluid. This tissue fluid
then moves into a system of
vessels that is known as the
lymphatic system. The lymphatic
system consists of lymph (a
milky fluid), lymph nodes,
lymph vessels, thymus, spleen,
tonsils and adenoids.
Lymph System
The lymph vessels form a one way drainage system from all parts
of the body back to a point near the heart where the cleansed
lymph fluid is drained back into the blood. The muscles that
surround the vessels squeeze the fluid in a one-way direction and
the presence of valves prevents the fluid moving backwards.
hls-herbs.com
Lymph System
At different points along the lymph vessels, there are structures
called lymph nodes that play an important part in the body’s
defence system.
Lymph System
If there is an infection in the tissues
the foreign particles, along with
dead cells and other debris, move
with the tissue fluid into the lymph
vessels. When they get to a lymph
node, the waste particles are filtered
off and any foreign particles are
destroyed by macrophages. Swollen
lymph nodes (glands) are a good
indicator of infection. The lymph
nodes also play an important role in
the third line of defence, as we will
learn more about later..
www.cancerresearchuk.org
Cell Death
Sometimes cells die to seal off an
area of tissue that is infected and
is not being successfully
defended by the body. If the
infected cells are surrounded by
a wall of dead cells it prevents
the infection from spreading to
other areas and infecting them.
www.pathguy.com
Cell Death
This wall of dead cells forms a capsule (known as a granuloma) or
cyst. The cells inside will then die, causing the destruction of the
pathogens that are infecting them. The debris inside the
granuloma or cyst will be destroyed by the macrophages that had
also surrounded the walled-off area. Granulomas form when a
person is suffering from tuberculosis or leprosy.
www.pathguy.com
Other Secretions
The body also produces special proteins that assist in the second
line of defence. Interferons are secreted by some cells when they
are infected with viruses. Interferons can cause nearby noninfected cells to produce their own antiviral chemicals, which
inhibit the spread of the virus. These interferons are non-specific
and are most effective in short-term viral infections such as colds
and influenza.
faculty.ccbcmd.edu
Other Secretions
The complement system is a
group of 20 proteins that assist
other defence mechanisms. These
complement proteins can be
involved in the destruction of
pathogens by stimulating
phagocytes to become more
active, attracting phagocytes to
the site of the infection or
destroying the membranes of the
invading pathogen.
www.nature.com
Homework
-Students to complete close passage at the bottom of 8.1.3
Second Line of Defence Worksheet.