4.1 Communicable Disease

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4.1 Communicable Diseases
Organisms that cause disease
Pathogen ­ a microorganism that causes disease
Bacteria Fungi Virus Protoctista
> Belong to the > Common > Viruses invade > Protoctista
kingdom fungal cells and take over usually cause
prokaryotae infections the genetic harm by
> Smaller than are where machinery and entering the
Eukaryote cells the fungi other organelles of host cells and
and reproduce lives in the skin of an animal and the cell. feeding on the
rapidly its hyphae which forms a > They reproduce contents as they
> Their mycelium grows under the skin by causing the host grow.
presence surface cell to replicate the > The malarial
causes harm by > The fungus can send out virus and when the parasite
damaging cells specialised reproductive hyphae host cell eventually plasmodium has
and releasing which grow out of the skin surface burst the many new immature forms
waste to release spores. viruses are released that feed on
products/toxin > The hyphae may release to infect healthy haemoglobin
s extracellular enzymes called cells. inside red blood
> In plants they cellulases to digest the > Influenza and cells and inhibits
often live in the surrounding host tissue Tobacco mosaic the organisms
vascular tissues > Black sigatoka and athlete's foot virus are examples ability to
> Tuberculosis are examples of this transport
and ring rot are oxygen around
examples in the blood
properly and
therefore slowly
prevents the
tissue cells from
Transmission of Pathogens
Lifecycle of a pathogen:
Travel from one host to another (transmission)
Entering the hosts tissues
Leaving the hosts tissues
Direct Transmission
Means of Transmission Factors that affect transmission
Direct physical contact such as touching a Hygiene ­ wash hands regularly
person who is infected or touching Keeping surfaces clean
contaminated surfaces including soil that Cleaning and disinfecting cuts
Sterilising surgical equipment

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HIV, bacterial Using condoms during intercourse
meningitis, athletes foot and ring worm
Faecal ­ oral transmission, usually by Treatment of waste water
eating/drinking water contaminated by the Treatment of drinking water
pathogen e.g. cholera Thorough washing of all fresh food
Careful food preparation and cooking
Droplet infection ­ pathogen is carried by tiny Catch it ­ bin it ­ kill it
airborne droplets e.g.…read more

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Indirect transmission of plant pathogen occurs as a result of insect attack.
The fungus that causes Dutch elm disease is carried by the beetle Scolytus multistriatus.…read more

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Plant Defences against pathogens
Passive Defences
Physical Defences
a) Cellulose cell wall ­ this acts as a physical barrier but also contains many chemical
defences that can be activated when a pathogen is detected
b) Thickening of the cell wall with lignin ­ lignin is a phenolic compound and
completely waterproof as well as largely indigestible
c) Waxy cuticles ­ these prevent water collecting on the cell surfaces which removes
the water that the pathogenic cells need to survive
d) Bark ­ most back also…read more

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Primary Defences against Disease
Primary defences are the defences in place that prevent pathogenic material from entering the
The Skin
The skin is the main primary defence. The outer layer of skin is called the epidermis and consists of
layers of keratinocytes. The keratinocytes are produces at the base of the epidermis and migrate
out to the surface of the skin, slowly drying out and their cytoplasm in replaced by the protein
keratin in the process of keratinisation.…read more

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Mucous Membranes
The epithelial layer contains mucussecreting cells called goblet cells and also mucus secreting
glands under the epithelium.
The mucus traps any pathogens that may be in the air
The epithelium is also ciliated
Cilia are tiny hairlike organelles that can move in a coordinated fashion to waft the mucus along.…read more

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Antibodies are produced by B lymphocytes,
and these neutralise foreign antigens. Long term disease protection is provided. An immunological
memory is produced as B memory cells are released and circulate in the body for a number of
1. Pathogen enters the body
2. The antigens on the pathogen are presented on the pathogens cell membrane as it travels
in the body fluids, on infected cells, and on the plasma membrane of macrophages that
have engulfed the pathogens during the secondary nonspecific response.
3.…read more

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B memory cells ­ cells that remain in the blood for a long time, providing longterm immunity
They are involved in the humoral response (producing antibodies)
Cell signalling
Macrophages release monokines which attract neutrophils (by chemotaxis ­ the movement of
cells towards a particular chemical) and stimulate differentiation of B cells (and the release of
T cells and macrophages release interleukins which stimulate clonal expansion (proliferation) and
the differentiation of B & T cells
Many cells release interferon which inhibits virus replication and stimulates…read more

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The tips of the y are the variable region but is the same for every type of antibody
A group of antibodies that bind to pathogen antigens and then act as binding sites for phagocytic
Some are nonspecific
Some are produced as part of a specific immune response and bind to specific antigens
Antibodies flag up a pathogen for the phagocyte/attach to antigen which has a use to the
pathogen, disabling it.
They also prevent the pathogen to enter the host cell.…read more

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Some antibodies bind to molecules that are release by pathogenic cells.
These molecules may be toxic and the action of antitoxins render
them harmless.…read more


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