Evolutionary Race

Biology Unit 4

Evolutionary Race

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Biology Unit 4
Revision Notes
Topic 6: Infection, Immunity and Forensics
16. Discuss how the theory of an `evolutionary race'
between pathogens and their hosts is supported by
the evasion mechanisms as shown by Human
Immunodeficiency Virus
As quickly as we evolve mechanisms to combat pathogens, they evolve new methods to overcome
our immune system. The bacterium which causes TB and the virus responsible for AIDS (HIV) have
both evolved features which help them to evade the immune system. The TB bacterium produces a
thick waxy coat which protects it from the enzymes of the macrophages. The protein coat of HIV is
constantly changing which means that the immune system can't target and destroy it.
The virus mutates rapidly so the antigens on the viral coat keep changing in the years after infection,
making it harder for the immune system to recognise the virus and so target and destroy it. The rate
of change only slows down as the T-cell count starts to fall seriously, when selection pressure on the
virus is reduced. Natural selection also favours mutations that enable to virus to replicate particularly
fast, allowing it to infect many cells as quickly as possible. By the time a vaccine has gone through all
its development and safety tests, the virus has changed and the vaccine doesn't work.
Text Book: p.123


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