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9.4 Search for better health: 4.Defence against
disease
| Syllabus reference (October 2002 version) | ||
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4. Often we recognise an infection by
the symptoms it causes. The immune response is not so
obvious, until we recover
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Students learn to:
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Students: |
Extract from Biology Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW
Background
Our bodies have three types of defence against pathogens.
The first consists of several barriers that prevent the entry
of micro-organisms. The second is the action of white blood
cells in destroying foreign particles, in a process called
phagocytosis. The third is carried out by the immune system,
which plays a complex role in targeting and destroying
pathogens as well as helping to make our bodies resistant to
them.
Our bodies contain very large numbers of bacteria (15% of
your body weight) and many of those in the intestine are
essential for our wellbeing. The collective term for all the
micro-organisms in our bodies, both the beneficial and
harmless ones, is microflora. An imbalance of
microflora in the gut can lead to disease symptoms such as
diarrhoea and malabsorption of nutrients.
gather, process and
present
information from secondary sources to show how a named
disease results from an imbalance of microflora in
humans.
Background
Pathogenic microflora usually attack specific parts of the
body. For example, the protozoan that causes malaria lives in
red blood cells, the bacterium that causes cholera lives in
the intestine, the bacterium that causes trachoma lives in
the eyes, one type of the Herpes virus is active in
cells around the lips and nose (causing cold sores) and
another type is active in cells in the genital area (genital
herpes).
- Choose a disease that results from an imbalance of microflora in humans, such as Crohn's disease or candida (thrush). Use biology textbooks, library references and, if possible, the Internet to gather information about the cause, symptoms and effects of the disease.
- To process the sources you find, assess their reliability by comparing the information provided. Look for consistency of information.
- It is appropriate for this syllabus point that you present your findings as a report. A scaffold and some language features of a report follow:
Scaffold for a report |
Features |
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Classification |
Classify the disease. Briefly preview the features to
be described. Use generalised terms. Use linking verbs
like is, has, becomes. Use present tense.
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Description
(Feature 1: e.g. cause)
(Feature 2: e.g. symptoms)
(Feature 3: e.g. effects)
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Crohn's disease 
National Institute of
Diabetes and Digestive and Kidney Diseases
identify defence barriers to prevent entry of pathogens in humans:
- skin
- mucous membrane
- cilia
- chemical barriers
- other body secretions
| Line of defence | Description | What it does |
|---|---|---|
| skin
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Skin continuously grows by new cells being
produced from below. Cells fit tightly together to form
a protective layer covered by dead cells.
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When unbroken, skin prevents the entry of
pathogens. Pores in the skin secrete substances that
kill bacteria.
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| mucous membrane
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cells lining the respiratory tract and
openings of the urinary and reproductive systems that
secrete a protective layer of mucus
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Mucus is sticky and traps pathogens and
other particles. When there are many pathogens more
mucus is produced to flush them out.
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| cilia
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Hair-like projections from cells lining
the air passages
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Move with a wavelike motion to push
pathogens from the lungs up to the throat.
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| chemical barriers
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acid in the stomach; alkali in the small
intestine; the enzyme, lysozyme, in tears
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Stomach acid destroys pathogens, including
those that are carried to the throat by cilia and then
swallowed. Alkali destroys acid resistant pathogens.
Lysozyme dissolves the cell membranes of
bacteria.
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| Other body secretions
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secretions from sweat glands and oily
secretions from glands in hair follicles
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Contain chemicals that destroy bacteria
and fungi.
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identify defence adaptations, including:
- inflammation response
- phagocytosis
- lymph system
- cell death to seal off pathogen
inflammation response
- Inflammation occurs when blood vessels around an infected area are supplied with extra blood. This makes the area swollen and red. The release of histamines by the damaged tissue increases the permeability of the blood vessels, which allows white blood cells to leave the blood vessels and move into the damaged tissue.
phagocytosis
- Some white blood cells, called macrophages and neutrophils, can very easily change their shape so that they flow around particles and completely enclose them within their cell, where they are broken up by cell enzymes. This is called phagocytosis.
lymph system
- The lymph system returns intercellular fluid to the blood system, filters cell debris and produces white blood cells responsible for the immune response.
cell death to seal off pathogen
- For some pathogens, macrophages and lymphocytes completely surround a pathogen so that it is enclosed in a cyst. The white cells involved die, so that the pathogen is isolated from its food supply and also dies.
identify
antigens as molecules that trigger the immune
response.
Background
If the previous two systems fail to destroy a pathogen
then the immune system comes into operation. It depends on
distinguishing between parts of the body and particles from
outside. It is important that phagocytes are able to make
this distinction, otherwise they would destroy body cells.
- Any molecule that the body recognises as being foreign is called an antigen. Antigens activate the immune response.
explain why
organ transplants should trigger an immune
response.
- Organs from another organism are recognised as foreign by the human immune system. The surfaces of the new organ contain antigens. These trigger an immune response and body attacks the new organ as if it were a pathogen.
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