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DEFENCE AGAINST INFECTION Bacteria are microscopic organisms. They come in many shapes and sizes, but even the largest
are only 10 micrometres long which is ten millionths of a metre.
Pathogens are microorganisms - such as bacteria and Bacteria are living cells and in favourable conditions they can multiply rapidly. Once inside the
viruses - that cause disease. Bacteria release toxins, and body, they release poisons or toxins that make us feel ill.
Diseases: Food poisoning, cholera, typhoid, whooping cough.
viruses damage our cells. White blood cells can ingest
and destroy pathogens. They can produce antibodies to
destroy pathogens, and antitoxins to neutralise toxins.
Viruses are many times smaller than bacteria. They are among the
smallest organisms known and consist of a fragment of genetic
In vaccination pathogens are introduced into the body material inside a protective protein coat. Viruses can only reproduce
in a weakened form. The process causes the body to inside host cells, and they damage the cell when they do this. A virus
produce enough white blood cells to protect itself can get inside a cell and, once there, can take over and make hundreds
against the pathogens, while not getting diseased. of thousands of copies fill the host cell and burst it open. The viruses
Antibiotics are effective against bacteria, but not against are then passes out in the bloodstream, the airways, or by other
viruses. Some strains of bacteria are resistant to routes.
antibiotics. Diseases: influenza (flu), colds, measles, mumps, rubella, chicken pox.
WHITE BLOOD CELLS White blood cells can:
The body has different ways of protecting itself against · ingest pathogens and destroy them
pathogens. The first defence is passive immunity. This is · produce antibodies to destroy pathogens
aimed at stopping the pathogen getting into the body in the · produce antitoxins that neutralize the toxins released by pathogens
first place. The body's passive immunity system includes the
skin, mucus and cilia in the respiratory system, acid in the LYMPHOCYTES
stomach, and enzymes in tears. If a pathogen still manages Pathogens contain certain chemicals that are foreign to the body and
to get into the body, the second defence takes over. This is are called antigens. Each lymphocyte carries a specific type of antibody
called active immunity, and the white blood cells have key - a protein that has a chemical 'fit' to a certain antigen. When a
functions in this. lymphocyte with the appropriate antibody meets the antigen, the
lymphocyte reproduces quickly, and makes many copies of the
PHAGOCYTES antibody that neutralises the pathogen.
Phagocytes can easily pass through blood vessel walls into Antibodies neutralise pathogens in a number of ways:
the surrounding tissue and move towards pathogens or · they bind to pathogens and damage or destroy them
toxins. They then either: · they coat pathogens, clumping them together so that they are
· ingest and absorb the pathogens or toxins easily ingested by phagocytes
· release an enzyme to destroy them. · they bind to the pathogens and release chemical signals to attract
Having absorbed a pathogen, the phagocytes may also send more phagocytes
out chemical messages that help nearby lymphocytes to Lymphocytes may also release antitoxins that stick to the appropriate
identify the type of antibody needed to neutralise them. toxin and stop it damaging the body.…read more

Slide 2

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VACCINATION antibiotic how it works
People can be immunised against a pathogen through vaccination. Different vaccines are needed for
different pathogens. penicillin breaks down cell walls
Vaccination involves putting a small amount of an inactive form of a pathogen, or dead pathogen, into
the body. Vaccines can contain: erythromycin stops protein synthesis
· live pathogens treated to make them harmless
· harmless fragments of the pathogen neomycin stops protein synthesis
· toxins produced by pathogens
· dead pathogens vancomycin stops protein synthesis
These all act as antigens. When injected into the body, they stimulate white blood cells to produce
antibodies against the pathogen. ciprofloxacin stops DNA replication
Because the vaccine contains only a weakened or harmless version of a pathogen, the vaccinated
person is not in danger of developing disease - although some people may suffer a mild reaction. If the
person does get infected by the pathogen later, the required lymphocytes are able to reproduce rapidly
and destroy it.
Vaccines in early childhood can give protection
against many serious diseases. Sometimes more than PENICILLIN
one vaccine is given at a time, like the MMR triple A bacterium damaged and distorted by
vaccine against mumps, measles and rubella. penicillin
Sometimes vaccine boosters are needed, because the The first antibiotic - penicillin - was
immune response 'memory' weakens over time. Anti- discovered in 1928 by Alexander Fleming. He
tetanus injections may need to be repeated every ten noticed that some bacteria he had left in a
years. petri dish had been killed by naturally
occurring penicillium mould.
Resistance Since the discovery of penicillin, many other
Bacterial strains can develop resistance to antibiotics. antibiotics have been discovered or
This happens because of natural selection. In a large developed. Most antibiotics used in
population of bacteria, there may be some cells that medicine have been altered chemically to
are not affected by the antibiotic. These cells survive make them more effective and safer for
and reproduce, producing even more bacteria that humans
are not affected by the antibiotic.
MRSA is methicillin-resistant staphylococcus aureus. Cleanliness
It is very dangerous because it is resistant to most One simple way to reduce the risk of infection is to maintain personal hygiene and to keep
antibiotics. It is important to avoid over-use of hospitals clean. In the 19th century, Ignaz Semmelweis realised the importance of cleanliness in
antibiotics, so we can slow down, or stop, the hospitals. However, although his ideas were successful, they were ignored at the time because
development of other strains of resistant bacteria. people did not know that diseases were caused by pathogens that could be killed.…read more


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