Vaccines, Herd Immunity, Active vs Passive
How a vaccine produces memory cells, how herd immunity protects a population, and the differences between active and passive immunity. Includes the 4-mark exam question, a worked model answer and the examiner trap on vaccines containing antibodies.
Chapters (8)
Exam question [4 marks]
A vaccine is given to most of the people in a town. Explain how this protects the people who are vaccinated and the people who are not. Then state one way in which passive immunity differs from the immunity the vaccine gives.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | The vaccine contains antigens (from the pathogen), which cause a primary response and the formation of memory cells | Needs antigens (or dead / weakened pathogen) AND memory cells formed; either one with "primary response" also earns the mark | "vaccine contains antibodies"; "vaccine contains the disease" with no mention of antigens or memory cells |
| 2 | On later infection, memory cells divide quickly into plasma cells, giving a faster, larger secondary response (antibodies), so the pathogen is destroyed before symptoms | "rapid", "more antibodies, sooner", "pathogen destroyed quickly" | "memory cells produce antibodies directly" (they divide into plasma cells); "antibodies stay in the blood" |
| 3 | Herd immunity: most of the population is vaccinated, so the pathogen spreads less / fewer carriers, so unvaccinated people are less likely to be infected | "less chance of contact with the pathogen", "pathogen cannot pass on easily" | "everyone is vaccinated"; "unvaccinated people become immune"; "unvaccinated people are protected" with no mechanism |
| 4 | Passive immunity: antibodies are given directly (injected or from mother), so no memory cells form and protection is short-lived (antibodies broken down) | "no memory cells", "temporary", "not made by own B cells"; "active: body makes its own antibodies, passive: antibodies come from outside"; "active is slower to start / longer lasting" | "passive immunity lasts a long time"; "passive immunity involves antigens" |
Marks 1 and 2 explain protection for the individual, mark 3 for the population, mark 4 is the contrast with passive immunity. Vaccines contain antigens, never antibodies. Herd immunity needs most, not all, of the population to be immune. Extra credit-worthy points that do not add marks beyond 4: herd immunity protects people who cannot be vaccinated (for example babies, or people with a weakened immune system).
Model answer (4/4)
The vaccine contains antigens from the pathogen, which cause a primary response in which B cells form plasma cells and memory cells. If the person is later infected, the memory cells divide quickly into many plasma cells, so a faster and larger secondary response produces antibodies and destroys the pathogen before symptoms appear. Because most people in the town are vaccinated, the pathogen spreads less, so unvaccinated people are less likely to meet it (herd immunity). In passive immunity, antibodies are given directly, so no memory cells form and the protection is short-lived.
Examiner trap
- Saying vaccines contain antibodies (they contain antigens)
- Not saying that passive immunity has no memory cells and is short-lived
- Saying herd immunity means everyone is vaccinated
Key terms
- vaccine
- herd immunity
- active immunity
- passive immunity
- memory cell
- antibody
- immunity
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:22What a vaccine contains
A vaccine contains antigens. These might be a dead or weakened pathogen, or just some of its surface proteins. The antigens cannot cause the disease, but they look foreign to your immune system. Antigens, not antibodies. That one word is a mark.
0:40How a vaccine builds memory
Your immune system has never met these antigens before. So it starts from scratch. After the injection, the antigen is picked up by a B cell with a complementary receptor. The helper T cells then stimulate that B cell. That cell is selected, divides, and makes plasma cells that release antibodies. This is the primary response, and it is slow and small. But some of the cells become memory cells, and they stay in your blood for years.
The antibodies fade away, but the memory cells remain. That is what gives you protection later. This is active immunity. Your own body makes the antibodies, and your own body makes the memory cells.
1:24When the real pathogen arrives
Later, the real pathogen arrives. Memory cells recognise the same antigen, divide quickly, and form many plasma cells. The secondary response is faster and larger, because there are more cells ready to respond and they start dividing at once. So the pathogen is destroyed before you feel ill.
That is how a vaccine protects the individual.
1:48Herd immunity
Now the population. If most people are vaccinated, then most people cannot catch the pathogen or pass it on. So there are fewer carriers, and the chance that an unvaccinated person meets one becomes very small. That is herd immunity. It protects people who cannot be vaccinated, such as babies or people with weakened immune systems.
If vaccination rates fall, the pathogen spreads again, and herd immunity is lost. Notice the word most. Herd immunity does not need everyone, and it does not mean everyone is vaccinated.
2:25Active versus passive immunity
Passive immunity is different. Here you receive ready-made antibodies from outside. A baby gets them from the mother across the placenta and in breast milk, or a patient may be injected with antibodies. Your own B cells were never involved, so no memory cells form. The antibodies are broken down after a while, so protection is quick but short-lived.
That is why it is used when someone needs protection straight away, and cannot wait for their own response. Active is slow but lasting. Passive is fast but temporary.
3:37Examiner trap and recap
The trap. Vaccines contain antigens, never antibodies. And passive immunity has no memory cells, and the antibodies are broken down, so it does not last. Recap. Vaccines make memory cells, which protects you. Enough vaccinated people protects everyone. Passive immunity is borrowed antibodies.
Next time, a virus that attacks the immune system itself. HIV, and how it causes AIDS.
Watch first
This lesson builds on:
- Primary and Secondary Immune Response · AQA 7402 · 3.2.4
Keep going
More from 3.2 Cells
- Antigens and Cell Recognition · AQA 7402 · 3.2.4
- Phagocytosis: How Phagocytes Work · AQA 7402 · 3.2.4
- T Lymphocytes: The Cellular Immune Response · AQA 7402 · 3.2.4
- B Cells, Clonal Selection and Antibodies · AQA 7402 · 3.2.4
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-046.