Types of immunity and the evolutionary race

Active and passive, natural and artificial

• Active immunity: your own immune system makes antibodies and memory cells. Takes time to develop but is long-lasting.
• Passive immunity: you receive ready-made antibodies from elsewhere. Immediate, but short-lived, because no memory cells are made and the antibodies are broken down.

Vaccination

A vaccine contains antigens that cannot cause the disease. A primary response produces memory cells, so a later infection triggers a fast secondary response. Boosters may be needed. If enough of a population is vaccinated, the pathogen cannot spread easily, which also protects unvaccinated people (herd immunity).

The evolutionary race

Hosts evolve better defences; pathogens evolve ways to evade them. Natural selection favours pathogens that avoid destruction:

• HIV: kills T helper cells; hides as a latent provirus; mutates rapidly, so its antigens keep changing (antigenic variation).
• Mycobacterium tuberculosis: survives inside macrophages and interferes with antigen presentation, so the immune system is slow to respond.
• Influenza: frequent changes in surface antigens mean memory cells from earlier infections or vaccines no longer recognise new strains, so new vaccines are needed each year.

Worked example

A snakebite victim is given antivenom. Classify this immunity and explain why it does not protect against a later bite.

1. Antibodies are injected: artificial passive immunity.
2. The person's own B cells are not activated, so no memory cells are made.
3. The injected antibodies are broken down within weeks.

Answer: Artificial passive immunity; no memory cells, so no long-term protection.

Check yourself

1. Vaccination gives which type of immunity?
   • Natural active
   • Artificial passive
   • Artificial active
   • Natural passive
   Show answer

   Artificial active. Antigens are given deliberately and the body makes its own antibodies and memory cells.

2. A baby gains antibodies from breast milk. What type of immunity is this?
   • Natural passive
   • Natural active
   • Artificial active
   • Artificial passive
   Show answer

   Natural passive. Ready-made antibodies received naturally.

3. Why does passive immunity last only a short time?
   • Too many memory cells are made
   • Antibodies multiply in the blood
   • No memory cells are made and the antibodies are broken down
   • It always triggers an allergy
   Show answer

   No memory cells are made and the antibodies are broken down. Without memory cells there is no lasting protection.

4. Which statement about active immunity is correct?
   • It is immediate and short-lived
   • It takes time to develop but is long-lasting
   • It involves injected antibodies
   • It never produces memory cells
   Show answer

   It takes time to develop but is long-lasting. The body must mount its own primary response.

5. Why is a new influenza vaccine needed most years?
   • The vaccine destroys memory cells
   • Influenza is a bacterium
   • Antibodies last only one day
   • The virus's surface antigens change, so old memory cells do not recognise new strains
   Show answer

   The virus's surface antigens change, so old memory cells do not recognise new strains. Antigenic variation lets the virus escape existing immunity.

6. What is herd immunity?
   • When animals in a herd make antibodies for humans
   • When enough people are immune that a pathogen cannot spread easily, protecting the unvaccinated
   • Immunity given by antivenom
   • When everyone catches the disease at once
   Show answer

   When enough people are immune that a pathogen cannot spread easily, protecting the unvaccinated. Breaking chains of transmission protects others.

7. Which is a way HIV evades the immune system?
   • It has a thick waxy cell wall
   • It is too large for phagocytes
   • It only infects plants
   • It hides as a latent provirus inside host cells
   Show answer

   It hides as a latent provirus inside host cells. Latent provirus is invisible to the immune system.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)
• CCBC Microbiology: HIV structure and life cycle
• Micro-Bites: How tuberculosis evades our immune response

Last updated 7 November 2026. Spotted a mistake? Tell us.