6.2.7Unit 6: Practical Skills in Biology II
Pathogens vs Hosts: An Evolutionary Race — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 13 of 48 Unit quiz
Section 1 of 5The Idea of an Evolutionary Race
Host evolution
- Vertebrates have evolved immune systems that deal with a wide range of pathogens.
Pathogen evolution
- Pathogens evolve ways of avoiding the immune system, so the host is placed at a disadvantage again.
Arms race
- Each side evolves new methods to gain an advantage over the other, in a continuing process.
Mechanism
- Variation arises by mutation, and natural selection favours the pathogens that avoid detection or destruction.
Selection pressure
- The immune system acts as a selection pressure, so pathogen variants that avoid it survive, reproduce and spread.
Section 2 of 5How HIV Evades the Immune System
How HIV Evades the Immune System
Kills helper T cells
- Destroying the cells that activate B cells reduces the ability of the body to detect it.
Antigenic variability
- A high mutation rate in antigen genes produces new strains that memory cells do not recognise.
New primary response
- Each new strain needs a fresh primary response, so protection is incomplete.
Hiding
- HIV prevents infected cells from presenting antigens, so T killer cells have difficulty targeting them.
Changing strains
- New strains of HIV appear during infection, so the immune system is always responding to a moving target.
Section 3 of 5How Mycobacterium tuberculosis Evades
Compare this with the way in which phagocytosis normally destroys bacteria.
1
The bacteria are engulfed by phagocytes in the lungs.
2
They produce substances that prevent lysosomes fusing with the phagocytic vacuole.
3
Therefore digestive enzymes cannot reach them, and they can survive and multiply inside the phagocyte.
4
They also interfere with antigen presentation by infected phagocytes.
5
The immune system finds it hard to recognise and destroy the infected cells.
Section 4 of 5Consequences
Disease persists
- Evasion explains why TB and HIV are difficult to cure or to prevent by vaccination.
Vaccine design
- Variation in antigens makes it hard to produce a lasting vaccine against HIV.
Selection
- Pathogens that evade immunity are more likely to survive and reproduce, so evasion alleles spread.
Evidence
- Evasion mechanisms are cited as support for the idea of an evolutionary race.
Section 5 of 5Exam Checklist & Tips
6.2.7 – Key Points for the Exam
- Evolutionary race The ongoing evolution of hosts and pathogens, each gaining an advantage over the other.
- Antigenic variability Frequent change in the antigens of a pathogen caused by mutation.
- Phagocytic vacuole A vesicle containing an engulfed pathogen, which lysosomes fuse with.
- Strain A genetic variant of a pathogen with different antigens.
- Antigen presentation Display of pathogen antigens on the surface of a cell to activate lymphocytes.
Common Mistakes
- Saying that pathogens change on purpose to beat the immune system;
- mutation is random and selection favours successful variants.
- Forgetting that memory cells for one strain do not recognise the antigens of another strain.
- Stating that TB bacteria avoid being engulfed; they are engulfed but survive inside.
- Missing the role of natural selection in explaining the evolution of evasion.
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