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In contrast
to the generally harmless forms of the common cold, misleadingly
known as "flu infections," influenza can lead to serious
illness. People who are already in poor health and have a weak immune
system are particularly at risk. This is the group of patients which
suffers the most fatalities. The symptoms include high fever, headaches
and pain in the joints, coughing, hoarseness and sore throat.
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Influenza
viruses
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Influenza
viruses spread easily. They are transmitted by airborne droplet
infection (sneezing, coughing) and by direct contract (shaking
hands, kissing).
These "travel-happy"
viruses appear everywhere in the world. They are masters of disguise
and change their shells every two to three years. This trick forces
the immune system to adjust to changes in influenza viruses over
and over again. This is why a previous attack of influenza does
not provide long-term immunity.
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New strains of
influenza come primarily from Asia. They form there in the spring and
reach Europe in the autumn. An early warning system spanning the globe
alerts health authorities every year if any new virus strains are on
the march. As a general rule, there is time to develop a vaccine for
the strains.
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Worldwide Influenza Epidemics
The "Spanish Flu" raged from 1918-1920 and struck
500 million people all over the world. This epidemic, the largest
ever, claimed 22 million lives. Other serious epidemics, all of
them originating in Asia, followed in 1957, 1968, 1989 and 1995.
In 1995, 20,000 people died from the consequences of an influenza
infection in Germany alone.
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Cytotoxic T cell
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Defending
Against Viruses
Viruses do not appear only outside of cells, where they run
a risk of being captured by antibodies. All viruses have one thing
in common. They penetrate cells and make use of their "copying
mechanism" for reproduction. The immune system has at its
disposal powerful weapons which can be deployed to prevent this
undesirable virus production: cytotoxic
T cells. They recognize cells infected by viruses and eliminate
them.
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If the defence process is to be initiated, an
antigen displaying cell, e.g., a phagocyte, must show virus
antigens on its surface. This is done with the help of the MHC-I
molecule. If a matching cytotoxic T cell docks on the "landing
pad," it is activated by a T
helper cell which recognizes the same antigen.
This grants the cytotoxic T cell a kind of "licence to kill."
If a cell is infected by viruses, it shows this by displaying
virus antigens on its surface, also with the help of MHC-I molecules.
So the cytotoxic T cells can find their targets and kill the infected
cell along with the viruses.
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The Masquerade of the Influenza Viruses
Flu is
caused by influenza viruses of the types A, B and C. Type A changes
especially frequently and is regarded as the primary cause of waves
of influenza. Most of the virus strains develop in Asia where people
and animals live together in extremely close quarters. Poultry and swine
are just as susceptible to the Influenza A viruses as humans, and they
can all infect each other. Under such conditions, new flu viruses can
develop very easily - either through antigen shift or antigen drift.
We speak of antigen
shift if a virus changes only a part of its surface antigens, but
also retains original antigens.
Animated
film
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An acquired
immunity against the original virus offers only limited protection
from the new strains because the antibodies and T cells formed
earlier find very few targets. The more thorough the masquerade
of the new virus is, the weaker the defence will be.
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We speak of antigen drift if the viruses redesign their surface
antigens completely. The original virus is completely unrecognizable
behind the new mask. Antibodies and T cells which fight the old antigens
are completely ineffective. Antigen drift occurs when two different
viruses invade the same cell. The genetic material of the two viruses,
which is in fragments, is recombined in new ways - often in the cells
of swine or birds. Flu viruses which are created by antigen drift are
especially dangerous because they often trigger serious, worldwide epidemics.
| In 1997, a poultry virus appeared
in Hong Kong. This influenza virus had no similarity whatsoever
with any previously known flu viruses. 25 people were infected,
six of them died. As a vaccine could not be produced fast
enough, the local health authorities decided to kill all 1.3
million chickens as a precautionary measure. |
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