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Biology – The Search for Better Health
Section 4: Often we recognise an infection by the symptoms it causes. The immune
response is not so obvious until we recover.

Identify defence barriers to prevent entry of pathogens in humans:
 Skin, Mucous membranes, Cilia, Chemical barriers, Body secretions.
The first line of defence is a non-specific defence and involves the body using both
physical and chemical barriers to prevent the entry of pathogens. The most
vulnerable areas on the body for the entry of pathogens are openings, such as
mouth and nose and the internal passages, such alimentary canal and urinogenital
canal. Barriers include:
- The skin – A physical barrier: The skin is the largest organ of the human
body and forms a tough outer barrier that covers the body and prevents
penetration by microbes. It is fairly dry, which helps to prevent the growth of
pathogens. The skin also contains its own population of harmless bacteria
that help to stop the invading microbes from multiplying. Oil and sweat
glands in the skin produce antibacterial and antifungal substances that
further inhibit the growth of invading pathogens. If the continuous barrier of
the skin is cut, the blood clots almost immediately to produce a temporary
patch to maintain the barrier until new skin forms.
- Mucous membranes: The respiratory, digestive, reproductive and urinary
tracts are covered with membranes that produce a thick layer of mucus,
which traps the entering pathogens. The pathogens are held in the mucus
until processes such as coughing and sneezing remove them. The mucus can
contain an antibody that prevents bacteria and viruses from attaching to the
surface. The mucus also provides a moist, nutritious layer in which the
harmless microbes live and produce substances that inhibit the growth and
entry of pathogens.
- Cilia: Cilia are tiny hairs that line the respiratory surfaces of the trachea and
bronchial tubes. The cilia are constantly beating in an upwards direction to
move the mucus containing the trapped pathogens the throat, where they
are removed by coughing sneezing or swallowing.
- Chemicals Barriers: Different types of chemicals secreted in different parts
of the body act as a barrier to the invading pathogens. In the alimentary
canal, pathogens entering with food or drink, or swallowed with mucus, will
be destroyed by the acidic conditions of the stomach or the alkaline
conditions in the intestines. The urinary and vaginal openings and the surface
of the skin are also acidic which inhibits the growth of pathogens.
- Other body secretions: Urine is sterile and slightly acidic. It helps to flush as
well as clean the ureters, bladder and urethra. It helps to prevent the growth
of microorganisms. Tears contain lysozymes that destroy the cell walls of
some bacteria. As the tears are produced and the eyelid blinks, the surface of
the eye is cleaned and the pathogens are washed away. This is also similar for
saliva as it cleans the teeth and lining of the mouth due to lysozymes.

Identify antigens as molecules that trigger the immune response.
An antigen is any molecule the body recognises as foreign and that triggers the
immune response. On the surface of the cells in the body, there are marker
molecules that identify the cell as belonging to the body. This protects the body from
attacking itself. 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.
The presence of these antigens causes the immune response to be activated to
destroy the foreign organisms. 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 contain a number of
responses that will be activated because the body recognises all these antigens as
foreign molecules.

Explain why organ transplants should trigger an immune response
When a person has an organ transplant, the new organ they are receiving from
somebody else has different antigens. The transplanted organ is therefore identified
as foreign and the immune response is activated to attack the organ in order to
defend the body.
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 less ‘violent’ immune response.
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.

Identify defence adaptations, including:
 Inflammation response, Phagocytosis, Lymph system, Cell death
When pathogens are successful in penetrating the barriers that are in place to
prevent their entry into the organism, non-specific responses that are ‘second line of
defence’ are quickly activated to try to destroy the invaders before they can cause
any damage to the body.
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. 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. 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 attach invading pathogens. Phagocytes are a special type of
white blood cell. Plasma also moves into the tissues, brining more phagocytes and
producing swelling in the area, forcing tissue fluid into the lymph and taking debris
and pathogens with it.
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 toxins and tissues repaired.
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 the
cell. Once it is inside the cell, enzymes are released to destroy the foreign material.
Phagocytes are specialised white blood cells or leucocytes. The two main types of
phagocytes are neutrophils and macrophages.
Neutrophils are the first to be called upon and move to the site of the infection,
inactivating pathogens. Neutrophils are short acting and then self-destruct after a
few days. They are used by the body to fight acute (short, severe) infections.
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, long lasting infections. After the macrophage has destroyed the foreign
particle, parts of the antigen are displayed on the surface of the macrophage. This
method is not always fully successful.
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 known as the lymphatic system. This consists of
lymph, nodes, vessels, spleen, tonsils and adenoids.
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. At different points
along the lymph vessels there are lymph nodes.
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.
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 a wall of dead cells surrounds the infected cells
it prevents the infection from spreading to other areas and infecting them. 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 has
also surrounded the walled off area.
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 non-infected 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.
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.

Gather, process and present information from secondary sources to show
how a named disease results from an imbalance of microflora in humans.
Disease: Candidiasis
The human body is home to a very large population and variety of microbes. They
live mainly on the skin, and in the intestines, the colon, the mouth and the vagina in
women. These microbes are known collectively as the microflora of the body. The
body supplies these microflora with the nutrients they require, and the conditions
they need to survive. In return, the presence of these microflora inhibits the growth
and multiplication of many pathogens that encounter the body, thus protecting the
body from contracting many diseases.
Candidiasis is a disease caused by the fungus Candida albicans. The fungus (yeast) is
part of the natural microflora of the human body and is normally present on the
mucous membranes of the female genital tract, mouth and respiratory tract and the
alimentary canal. The number of C. albicans is usually kept low by competition from
other micro-organisms in the body. If the natural balance of the microflora is upset
for any reason, the number of C. albicans increases and the disease develops.
One of the ways the natural balance of microflora can be upset is by taking
antibiotics to treat a bacterial infection. As well as reducing the number of
pathogenic bacteria in the body, the antibiotics could also reduce the number of
bacteria in the natural population of microflora. This allows C. albicans to multiply in
an uncontrolled way and allows the disease candidiasis to become established.
Other factors that could allow the increase in the number of C. albicans include: the
suppression of the immune system, diabetes, steroid use, pregnancy, oral
contraceptives, general illness and intravenous drug use such as heroin.
The symptoms of Candidiasis include:
- Vaginal discomfort – itching or burning
- A thick white discharge that has a yeasty smell
- Redness or swelling of the genital tract
- Stinging or burning while urinating or during sex
- Splits in the genital skin.
Treatment methods include:
- Antifungal creams or vaginal pessaries (tablets) – these are put inside the
vagina with a special applicator and are used from one to six days, depending
on the instructions. There are also oral tablets – but these should not be used
as first line treatment.
- The best treatment is prevention:
- Avoid using soap in genital areas – use sap substitutes.
- Avoid perfumed toilet paper
- Avoid synthetic or tight fitting pants.