Bài tập 27
You will hear a lecture on immunity and immunisation.
First, you have some time to look at questions 31 to 40. Listen carefully and answer questions 31 to 40.
Good afternoon and thank you for your warm welcome. This will be the first talk in a series of five on Health interventions - protection and prevention. Could I start by asking for a show of hands - how many of you had a flu vaccination at the beginning of winter? Mmm ... I thought so. You young ones always think you're indestructible.
Well, as you are no doubt aware disease-spreading germs or pathogens are everywhere. On a daily basis the human body has to ward off attacks by various harmful bacteria and viruses. A healthy body has a good defense system against many of these germs but the defense only operates well against micro-organisms that it has already encountered, in which case it is said to be immune.There are two ways in which humans acquire natural immunity: actively, when a person has first suffered and then recovered from an illness, and passively, when ready-made protection is transferred into the body, for example, from the maternal blood via the umbilical cord to an unborn child, or through breast milk.
Now, artificially acquired immunity can help the body to fight disease, so we can use active immunisation as a preventative measure. This is when a person is vaccinated against an illness by injection or oral ingestion of a tiny amount of weakened or inactive germs - not enough to actually cause him or her to contract the illness but sufficient for the body's defence system to recognise and respond to the threat by forming antibodies.
Intervention using passive immunization, on the other hand, is a method of curing an illness after it is too late for prevention. It is less effective than active immunization and takes longer to work. It is used when the body has already been invaded by bacteria and the person is ill. In this case there is no time for the body to make antibodies of its own so proteins - usually taken from the blood of animals - are injected to equip the patient with the essential antibodies to combat the particular illness.
Let's have a quick look at a bit of history: The discovery of vaccination to boost the body's immune system by making it sensitive to particular disease-causing bacteria was made by an eighteenth-century English doctor called Edward Jenner. He noticed that survivors of smallpox, a common but extremely dangerous disease, never contracted the disease a second time. In other words, they were immune. He studied a similar disease in cows called cowpox and realized that people in contact with the infected cows became ill with symptoms resembling smallpox. However, this disease was quite mild by comparison and those who contracted cowpox were then immune to smallpox. He conducted an experiment by injecting a child with a small amount of pus taken from a cowpox pustule. The child subsequently became ill but soon recovered. Later, he injected the child with pus from a smallpox pustule and the child did not get sick. He had developed immunity to the more dangerous disease. The antibodies produced to fight the cowpox bacteria had been able to fight off the smallpox bacteria.
What are antibodies? Well, antibodies are made by white blood cells called B-lymphocytes. And this is done in response to the presence of antigens, or other bacterial toxins, which have been released by the micro-organisms (what we commonly refer to as germs') that have invaded the body. These Y-shaped antibodies (or you can think of them as 'anti-toxins') may stop the toxins or repair the damage they have done by what is known as the antigen-antibody reaction which takes place within the plasma of the blood. A correct antibody (for that disease) clings to a particular antigen in order to render it harmless. Large numbers of these pairs clump together to form a bigger unit. This is called agglutination and is able to be seen by the naked eye which is very helpful for doctors and other specialists to determine which illnesses a patient is immune to.
Inoculation, or active vaccination, can protect people from serious diseases. The vaccine may make a person feel unwell for a few days when the immune system starts to produce antibodies to match the introduced antigen. This is called a primary reaction. If that particular antigen should ever enter the body again later, a secondary reaction takes place. The body is then able to produce large numbers of corresponding antibodies within a short time so the invading antigens are quickly wiped out without the person suffering any harm from the disease.
That is the end of section 4. You now have half a minute to check your answers. That is the end of the listening test. You now have 10 minutes to transfer your answers to the listening answer sheet.