Three Lines of
Defence
Protecting the body against
infection
First Line of Defence – Skin
•The first line of defense provides physical
barriers for the pathogen, such as our unbroken
skin.
•The skin provides a protective, mechanical
barrier against infection due to its layer of keratin
which microorganisms cannot penetrate.
•It protects our internal organs and tissue from
having direct contact with environmental
pathogens.
•The skin provides niche for non-harmful
pathogens to live. These pathogens – mostly
bacteria – can inhibit the growth of harmful
bacteria.
•The dryness of the skin also prevents growth of
http://www.elwoodstudio.com/key_pages/sub/skin.html
pathogens.
First Line of Defence – Fluids
•Mucous membranes occur along the alimentary canal, urogenital
canal, reproductive tract and respiratory surfaces. These membranes
produce mucous which traps the pathogens. The mucous contains the
antibody IgA which prevents viruses and bacteria from attaching.
•Oils are secreted from oil glands in the skin. They can inhibit the
growth of some bacteria and fungi.
•Tears contain lysosomes that help destroy the cell wall of some
organisms
•Saliva can also destroy or inhibit pathogens with lysosomes.
•Sperm contains compounds that act against bacteria.
•Urine flushes the lower urinary tract as it is sterile and acidic.
First Line of Defence - Cilia
http://www.clevelandclinic.org/health/wordofmouth/
•Cilia are like tiny hairs that line the
airways and trap pathogens. They
sweep back and forth to help move
pathogens trapped in mucous back
up the airways so that they can be
coughed up, sneezed out or
swallowed.
•Eye lashes are not cilia, but
pathogens can also get caught in
them and therefore not enter the eye.
http://pediatrics.med.unc.edu/div/infectdi/pcd/home.htm
First Line of Defence - Chemicals
•Chemical barriers provide conditions that are not
hospitable for microorganisms.
•The pH of the skin, which is slightly acidic, can prevent the
growth of some pathogens.
•The skin secretes salt, which also inhibits bacterial growth.
•The pH in the stomach can destroy many pathogens.
•Acids in the vagina inhibit growth of fungi and bacteria in
the female reproductive tract.
Second Line of Defence
•If a pathogen makes it past the first line of defense, it enters
into the body and initiates the immune response.
•The second line of defense consists of an internal and nonspecific type of defense.
•The actual immune response is triggered by an antigen.
•An antigen is a marker protein that is located on the surface of
bacteria and viruses, or is part of a toxic molecule (such as
venom). The body recognizes the foreign antigen as ‘non-self’.
•Each pathogen has its own antigen.
Second Line of Defence Phagocytosis
•Phagocytes are white
blood cells (lymphocytes)
that engulf and destroy
foreign microorganisms.
•Macrophages are typical
phagocytes. They are
large, mobile, long living
and filled with lysosomes.
http://www.mnsfld.edu/~mcb/
Second Line of Defence Inflammation
•When a pathogen enters our body an inflammation response is triggered
by the damaged tissue.
•Blood vessels dilate so that the blood flow is increased to the site of
infection. This raises the temperature at the site of infection.
•The increased blood flow brings an increased number of white blood cells
and helps contain the infection.
Second Line of Defence –
Lymph System
•The lymph system consists of lymph
nodes, lymph capillaries and fluid lymph.
•The lymphatic system collects fluid from
the blood vessels and tissues and filters it
of foreign bodies.
•The lymph nodes produce white blood
cells that are added to the lymph as it
flows towards the heart. The nodes act as
filters.
•Valves prevent the back flow of lymph
and muscular contractions assist
movement along the lymph capillaries
where it reenters the circulatory system
near the heart.
http://www.sirinet.net/~jgjohnso/index.html
Second Line of Defence –
Cell Death
•Cells at the site of a wound start to die in order to seal off
the area and help prevent further infection.
•Programmed cell death is known as apoptosis.
Second Line of Defence –
Other secretions
•Virus infected cells have been found to produce chemicals
called interferons that induce nearby non-infected cells to
produce antiviral enzymes.
•These antiviral enzymes block the translation of the viral
messenger RNA into a polypeptide. The virus therefore
cannot reproduce.
•Interferons are most helpful when fighting short term viral
infections such as colds and influenza.
•Today, interferons are commercially available using DNA
technology.
Third Line of Defence
•Third line of defense consists of a specific immune response
and is the bodies main immune response.
•It helps the body to build up a resistance and fight long term
infections.
•This line of defense comprises of antibodies, T cells and B
cells.
•Antibodies are proteins produced in the lymph that bind to
specific antigens and destroy them.
•T cells and B cells are lymphocytes that are activated by the
presence of antigens.
Third Line of Defence - Antibodies
•Antibodies (also called immunoglobulins) are receptor proteins located in
the blood plasma (and other body fluids) that combine and neutralise an
antigen.
•They are highly specific for the particular antigen that stimulated their
synthesis and release.
•The antigen-antibody complex activates a series of proteins that destroy
the bacteria and release histamine (that attracts phagocytes) that causes
an inflammatory response.
Antibody
Antigen
Bacterial cell
Antigen/
antibody
complex
Third Line of Defence
http://www.nearingzero.net/index.html
Third Line of Defence - Antibodies
•Antibodies can react in a number of ways:
•They can combine with the antigen to cover it’s active site.
•It may dissolve parts of the cell wall.
•It can neutralize the toxins produced by the antigen.
•IgA causes the bacteria to clump together and become less
active so that it can be taken more easily to the lymph.
•It can make the antigen more susceptible to phagocytosis.
Third Line of Defence – T Cells
•T cells (or T lymphocytes) are types of non-phagocytitic white blood cells
that produce a ‘cell mediated response’ and destroy antigens.
•They derive from bone marrow stem cells that migrate to the thymus to
become T cells. They circulate through the blood, lymph and spleen and
are inactive until they come into contact with an antigen.
•The antigen combines with the antibody and activates it to make copies of
itself.
•Once activated, some T cells secrete substances that can directly destroy
antigens, others cause the bacteria to clump together so they can be taken
more easily to the lymph, while others stimulate B cells and macrophages.
•After an infection, some T cells remain in the body as memory cells ready
incase a similar infection of the same pathogen occurs.
http://www.acm.uiuc.edu/sigbio/project/updated-lymphatic/lymph8.html
Third Line of Defence – T Cells
Third Line of Defence – T Cells
•Killer T Cells, or Cytotoxic T cells (Tc) secrete chemicals that destroy
antigens directly or inhibit replication of viruses. They also enhance
macrophage activity. Cytokines help co-ordinate B & T cells. Natural killer
cells are a type of cytotoxic T cell that destroys abnormal host cells such as
cancer cells or those infected by viruses.
•Helper T Cells (Th) secrete interleukins (hormone like messenger
chemicals between lympocytes) which enhance antibody production by B
cells and helps regulate cytotoxic T cells . HIV infects T4 helper cells which
reduces their number and function. They can also stimulate macrophages
for phagocytosis.
•Memory T Cells retain the ability to recognise the original invading antigen.
Future infections can be dealt with much quicker and more effectively as
they can produce large numbers of antibodies very quickly.
•Suppressor T Cells help turn off the immune response once the antigen is
destroyed and the infection is under control.
Third Line of Defence – B Cells
•B cells are non-phagocytitic white blood cells and provide the ‘antibody
mediated’ response.
•They derive from bone marrow stem cells that stay and develop in the
bone marrow and later accumulate in the spleen and lymph nodes.
•The presence of antigens stimulates the lymphocytes to divide by
mitosis to clone themselves. They then differentiate into plasma cells that
send antibodies into the blood, or into memory cells. This usually occurs
in the lymph node and causes it to swell when the body is fighting an
infection.
•The plasma cells secrete immunoglobulins/antibodies with a shape that
is compatible with the antigen in order to form the antibody-antigen
complex.
Third Line of Defence –
B & T Cell Interaction
•Once an antigen enters the body it is ingested and processed by a
macrophage. When engulfing an antigen, macrophages display
fragments of the antigen on their outer membrane which B & T cells
recognize.
•B & T cells can collaborate because they are in close proximity and
often make contact with each other.
•They are also both regulated by cytokines. Cytokines are proteins
secreted by T cells and macrophages that signal to other cells to initiate
the immune response (such as B cells transforming to plasma cells).
•B cells cannot function without T cells that stimulate them to produce
antibodies, and helper T cells induce B cells to divide and produce large
numbers of clones.
Antibiotics
•The first antibiotic – penicillin - was discovered by Howard
Florey and Alexander Fleming who won the Nobel Prize in
1945 for their work.
•Penicillin is derived from the Penicillium mould and after
becoming commercially available in 1941 saved the lives of
many people with serious bacterial infection.
•There is now a wide range of antibiotics available and they
all inhibit or kill bacteria in a variety of ways. Some destroy
cell walls, others destroy cell membranes, while others
interfere with cellular processes such as protein synthesis.
•Some antibiotics are ‘broad-spectrum’ and act on a wide
range of bacteria, while others are narrow-spectrum and only
act on one or two bacteria.
http://www.livestockconcepts.com/LA200.html
Second Line of Defence –
The Immune Reponse
•The immune response is the reaction of an organism to either prevent or
overcome an infection caused by an invading pathogen.
•The body recognises the foreigners (by their antigens) and tries to destroy
them.
•Organisms can be born with a natural immunity to a pathogen and therefore
will not be affected by it.
•Humans can acquire immunity passively by being given the antibodies via a
vaccine. This type of immunity only lasts a few weeks as the person does not
make their own antibodies. Babies can acquire natural passive immunity via
the mothers placenta.
•Acquired immunity can also be active via exposure to antigens. This type of
immunity can occur naturally if the person is exposed to the pathogen and
produces their own antibodies, or through vaccinations where the antigen is
introduced to the persons immune system artificially.
Vaccination
•Vaccination is the introduction (orally or intravenously) of an attenuated
(less virile) or dead pathogen that induces an immune response. B cells
produce the antibody to destroy the antigen, and T memory cells develop to
prepare the body for future infections.
•If memory cells decrease in numbers over time, booster vaccinations can
be given.
•Vaccines do not cause the disease as the pathogens are not alive.
•Before 1700s the smallpox virus killed in pandemic (worldwide)
proportions. In 1797 Edward Jenner noticed that milkmaids did not contract
the disease. He injected a young boy with cow pox and then with smallpox
and noted that he did not contract the disease. The first innoculation
(developed from the sore of a milkmaid with cowpox) against a deadly
disease had been discovered. The WHO introduced this vaccine worldwide
in the1960s and has now eradicated smallpox.
•Today we vaccinate children against bacterial diptheria and viral polio.
Organ Transplant
•Organ transplant is being used more and more to treat patients whose own
tissues no longer function sufficiently. Organs that can be transplanted
include the kidney, liver, heart, lung and bone marrow.
•The introduced organ contains antigen proteins that are recognised as
foreign to the host body which leads to the stimulation of an immune
response that could attack the new tissue.
•Tissue transplant should be carried out between individuals who are
genetically similar for the tissue proteins – that is, they have a high MHC
(major histocompatibility). Identical twins for example are completely
compatible.
•Drugs that suppress the immune response are necessary so that the host
does not reject the new organ.
•The obvious problem with this is that, with a suppressed immune system,
the host is prone to the dangers of any infection. The use of suppressive
drugs must be balanced with the risk of life threatening infection.
References
•American Lung Association of Ohio and Cleveland Clinic Health System (2000)
Youth Tobacco Protection Program Retrieved from site
http://www.clevelandclinic.org/health/wordofmouth/ May 2004.
•Aubusson, P. and Kennedy, E. (2000) Biology in Context. The Spectrum of Life
Oxford University Press, Melbourne, Australia.
•Board of Studies (2002) STAGE 6 SYLLABUS Biology Board of Studies, NSW,
Australia.
•Carson, J. (2000) University of North Carolina Paediatrics Department. Retrieved
from site http://pediatrics.med.unc.edu/div/infectdi/pcd/home.htm May 2004.
•Humphreys, Kerri (2002) The Search for Better Health. Science Press, Australia.
•Johnson, J. (2004) The World of Biology Retrieved from site
http://www.sirinet.net/~jgjohnso/index.html May 2004.
•Kinnear, J and Martin, M (2001) Biology 2 HSC Course: Jacaranda HSC Science
John Wiley & Sons, Australia, Ltd.
References
• Kim, Nick [no date] Nearing Zero retrieved from website
http://www.nearingzero.net/index.html January 2004.
•Mudie, K. et.al. (2000) Heinemann Biology Malcom Parsons, National Library of
Australia, Australia.
•Sternik, J. (2002) Cell and Molecular Biology. Mansfeild University. Retrieved from
site http://www.mnsfld.edu/~mcb/ May 2004.