The evolutionary race between pathogens and hostsSpec 6.12
In short
The evolutionary race theory states that hosts and pathogens are in a continuing cycle of selection: mutations that help a pathogen evade defences are selected, then hosts with improved resistance are selected. Evasion mechanisms such as antigenic variation, hiding inside cells and suppressing immunity support the theory.
Pathogens have short generation times and high mutation rates, so they evolve quickly. A pathogen that survives host defences reproduces and passes on the allele. Hosts with a more effective immune response also survive and reproduce, so their offspring are better defended. The pressure from each side selects for change in the other: an evolutionary race. Evidence is shown by the many ways pathogens evade the immune system.
| Mechanism | Example | Why it works |
|---|---|---|
| Antigenic variation: changing surface antigens | Influenza virus (mutations and reassortment give new surface proteins); HIV (reverse transcriptase makes many errors); Trypanosoma (switches its surface protein) | Memory cells and antibodies for the old antigen no longer recognise the new type, so there is no secondary response and a new vaccine may be needed |
| Hiding inside host cells | HIV (inside helper T cells, as a latent provirus); Mycobacterium tuberculosis (inside macrophages); malaria parasite (liver and red blood cells) | Antibodies cannot reach the pathogen inside the cell, and few or none of its antigens are displayed on the host cell surface |
| Killing or disabling immune cells | HIV destroys helper T cells | Fewer B cells and killer T cells are activated, so the immune response collapses |
| Preventing digestion by phagocytes | M. tuberculosis stops lysosome fusion; some bacteria have a capsule | Pathogen survives phagocytosis, or is not engulfed at all |
These mechanisms are examples of adaptations produced by natural selection. Because the pathogen population keeps producing new variants, the host immune system, vaccines and drugs always have to respond to a moving target.
To support the theory, name the mechanism, give the organism, and link it to the host: 'HIV mutates rapidly, so new antigen variants are not recognised by memory cells'.
Quick check
How does lysozyme kill bacteria?
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It hydrolyses peptidoglycan in the cell wall, so the bacterium bursts by osmosis.
How does interferon protect a neighbouring cell?
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It switches on genes for enzymes that break down viral mRNA and block protein synthesis, so the virus cannot replicate.
Which cell secretes antibodies?
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The plasma cell (B effector cell).
What is the difference between natural passive and artificial passive immunity?
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Natural passive: antibodies pass from mother to baby. Artificial passive: ready-made antibodies are injected.
Why does antigenic variation help a pathogen survive?
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Memory cells and antibodies for the old antigen do not recognise the new antigen, so there is no fast secondary response.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026