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MODERN SCIENCE
1BACH
UNIT4:LIVE MORE, LIVE BETER
INFECTIOUS DISEASES AND THE IMMUNE
SYSTEM
The body’s first line of defence against harmful pathogens is the skin
and stomach acid, the second is white blood cells. Vaccination involves
exposing the body’s immune system to a weakened or harmless
version of the pathogen in order to stimulate white blood cells to
produce antibodies.
Pathogens
Pathogens are microorganisms that cause infectious disease. Bacteria and
viruses are the main pathogens.
Bacteria
Bacteria come in many shapes and sizes, but
even the largest are only ten micrometres long
(ten-millionths of a metre).
Bacteria are living cells and can multiply rapidly in
favourable conditions. Once inside the body, they
release poisons or toxins that can make us feel ill.
Viruses
Viruses are many
times smaller than
bacteria. They are
among the
smallest organismsk
Salmonella bacterium cell
nown, consisting of
a fragment of
genetic material
inside a protective
protein coat.
Viruses can only
reproduce inside
host cells, damaging A hepatitis C virus showing DNA
them when they do
enclosed in a protein coat.
so. Once inside, they
take over the cell and make hundreds of thousands of copies of themselves.
Eventually, the virus copies fill the whole host cell and it bursts open. The
viruses then pass out through the bloodstream, the airways, or by other
routes.
Population growth of microorganisms
In the correct conditions (with warmth, moisture, nutrients) bacteria can
multiply rapidly. The human body can provide these conditions for bacteria to
multiply, for example in a cut. Each bacterium splits into two up to every 20
minutes.
So, after one hour a single bacterium could have reproduced to give eight
bacteria.
microorganisms reproduce quickly in the human body
Lines of defence
When microorganisms enter the body, they release toxins. The toxins damage
cells to cause the symptoms of the disease. The body’s first line of defence is
its natural barriers which include:
skin
chemicals in tears
chemicals in sweat
stomach acid.
White blood cells
The body’s first line of defence is called passive immunity, which means
preventing the pathogen from entering in the first place. If a pathogen
manages to get into the body, the second line of defence takes over, which is
calledactive immunity. The white blood cells have key functions in this.
Functions of the white blood cells
White blood cells can:
ingest pathogens and destroy them
produce antibodies to destroy pathogens
produce antitoxins that neutralise the toxins released by pathogens.
In a written examination, it is easy to get carried
away with metaphors about invaders and battles:
stick to the point. Note that:
the pathogens are not the disease,
theycause the disease.
white blood cells do not eat the pathogens,
they ingest them.
antibodies and antitoxins are not living things,
they are specialised proteins.
Memory cells
A white blood cell ingesting disease-
Memory cells are a type of white blood cell that
can respond quickly when it meets a
microorganism for the second time. They produce
the right antibody for the particular microorganism and destroy it before you
feel unwell. This is described as being immune to a disease.
causing bacteria.
Mutation rate
On average an adult human will catch two to four colds every year, and a child
will catch even more (because their immune system is still developing. This
happens for two main reasons:
there are hundreds of different cold viruses
the viruses have a high mutation rate
A high mutation rate means that the DNA of the virus frequently changes, so
the human body has to destroy the virus using a different antibody each time,
which makes it harder to develop immunity.
Types of white blood cell
There are several types of white blood cell, each with a different function. But
there are two main groups. The first of these groups surrounds and digests
bacteria. They can pass easily through blood vessel walls into the surrounding
tissue, and move towards pathogens or toxins.
The second produces antibodies to label microorganisms. Pathogens contain
certain chemicals that are foreign to the body. These are called antigens. Each
of this group of white blood cell carries a specific type of antibody - a protein
with a chemical ‘fit’ to a certain antigen. When a white blood cell with the
appropriate antibody meets the antigen, the white blood cell reproduces
quickly to make many copies of the antibody that neutralises the pathogen.
http://www.bbc.co.uk/schools/gcsebitesize/science/21c/keeping_healthy/disease_resistancerev4.sht
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