Cambridge International A Level Biology Revision Guide

Chapter 11: Immunity
Smallpox was first; polio second?
You must have been vaccinated against polio. You may
wonder why people make such a fuss about having
a vaccination against a disease that you are unlikely
ever to come across. There may be very few cases of
polio now, but in the past large numbers of people
contracted the disease. In many cases, the disease was
mild, but the virus that causes polio can infect the base
of the nerves and cause paralysis. This happened in
about 1% of all cases.
Vaccines against polio became available in the
1950s and immediately mass vaccination programmes
began. These have proved very successful: the last case
of polio in the Americas was in 1991. In 1994, it was
declared that transmission of polio had been broken so
that the disease was no longer endemic in the Western
Hemisphere. Success has been slower coming in the
rest of the world. In 2013, there were only 369 cases of
polio throughout the world. Most of these occurred in
three countries: Nigeria, Somalia and Pakistan.
India has mobilised huge numbers of medical
staff and around 230 000 volunteers to vaccinate
children throughout the country (Figure 11.1). This
didn’t just happen once, but on many occasions in the
government’s drive to eradicate polio. Vaccination
programmes in the other countries where polio
remains endemic have not been as successful as in
India. One reason is that some people have resisted
attempts by medical staff to vaccinate the local
population. Some medical staff have been attacked,
even killed, while working on polio eradication
campaigns in Pakistan.
In 1980, the World Health Organization announced
that the viral disease smallpox had been eradicated
as there had not been a new case anywhere in the
world for the previous three years. Polio is the most
likely candidate for the second infectious disease
to be eradicated. You can follow the progress of the
campaign by searching online for ‘polio eradication’.
Figure 11.1 On 15 April 2012, 170 million children across India
were vaccinated against polio as part of the government’s
programme to eradicate the disease.
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We now consider in detail something that was mentioned
in Chapter 10: our ability to defend ourselves against
infection by pathogens (disease-causing organisms).
We have seen that some people experience few or no
symptoms when exposed to certain infectious diseases.
Even though a person may be a carrier of disease to other
people, he or she has immunity. How is this possible? The
disease measles is used as an example here.
Most people have measles only once. In most cases,
it is very unlikely that anyone surviving the disease will
have it again. They are immune. Their body’s internal
defence system has developed a way of recognising the
measles virus and preventing it from doing any harm
again. Immunity is the protection against disease provided
by the body’s internal defence or immune system. The
external cellular and chemical barriers that protect us
from infection are part of that defence system.
Defence against disease
External defence system
We have a variety of mechanisms to protect ourselves
against infectious diseases such as measles and those
described in the previous chapter. Many pathogens do
not harm us, because, if we are healthy, we have physical,
chemical and cellular defences that prevent them entering.
For example, the epithelia that cover the airways are an
effective barrier to the entry of pathogens (pages 187–189);
hydrochloric acid in the stomach kills many bacteria that
we ingest with our food and drink; blood clotting is a
defence mechanism that stops the loss of blood and prevents
the entry of pathogens through wounds in the skin.