Primary and Secondary Immune Response
How plasma and memory cells explain the slow first antibody peak and the faster, larger second response, with graph interpretation. Includes the 4-mark exam question, a worked model answer and the examiner trap on memory cells secreting antibodies.
Chapters (8)
Exam question [4 marks]
A person is infected with a pathogen and, several weeks later, is infected with the same pathogen again. Explain why the second immune response is faster and produces a higher concentration of antibodies than the first.
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 | In the first response, the B cell with the complementary receptor is selected and divides (clonal expansion) into plasma cells that secrete antibodies (minimum for the mark: selection or division AND plasma cells secrete antibodies; "takes time" is optional) | "clonal selection", "B cell divides by mitosis", "plasma cells secrete antibodies", "slow because few B cells fit", "first response needs selection of the specific B cell" | "B cells make antibodies" with no plasma cells; "antigens make antibodies" |
| 2 | Some of the clone become memory cells, which stay in the blood for a long time | "memory cells remain / persist", "long-lived" | "antibodies stay in the blood for years"; "memory cells secrete antibodies" |
| 3 | At the second infection, memory cells recognise the antigen and divide / differentiate into plasma cells more quickly | "faster clonal expansion", "shorter lag phase", "no need for selection again" | "the body remembers the pathogen" alone; "memory cells change into antibodies" |
| 4 | More plasma cells are made, so more antibodies are secreted, giving a higher concentration | "larger number of plasma cells", "higher peak" | "more antibodies" with no reference to more plasma cells or memory cells |
Do not award both points 3 and 4 from one clause unless both speed and amount are stated. Mark points 1 and 2 describe the primary response, 3 and 4 the secondary. Named cells are needed: "the body remembers" scores nothing. Memory cells never secrete antibodies; plasma cells do. Extra credit-worthy points that do not add marks beyond 4: the pathogen is destroyed before symptoms develop.
Model answer (4/4)
In the first infection, the B cell with the complementary receptor is selected and divides by mitosis (clonal expansion) into plasma cells that secrete antibodies, which takes time. Some of the clone become memory cells, which stay in the blood for a long time. At the second infection, memory cells recognise the antigen and divide and differentiate into plasma cells much more quickly. More plasma cells are made, so more antibodies are secreted and the concentration is higher.
Examiner trap
- Saying memory cells secrete antibodies
- Saying antibodies stay forever
- Not explaining why the secondary response is faster and larger
Key terms
- plasma cell
- memory cell
- primary response
- secondary response
- antibody concentration
- clonal expansion
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:25Drawing the primary response
Let's draw what happens. Time along the bottom, antibody concentration up the side. At the first infection, nothing much happens at first. The line stays flat. Then antibody concentration climbs to a small peak, and falls again. This is the primary response. It is slow, and it is small.
0:46Clonal selection and plasma cells
Why so slow? Only a few B cells have a receptor that fits this antigen. The one that fits is selected, and divides by mitosis to make a clone. That takes time. Most of the cells in the clone become plasma cells. Plasma cells secrete lots of antibodies, and those antibodies are what you see rising on the graph.
Remember, each B cell carries a receptor with one particular shape. Only the B cell whose receptor is complementary to this antigen is selected. This is called clonal selection. And the dividing that follows, to make lots of identical cells, is called clonal expansion. But some of the clone become something different. Memory cells.
1:31Memory cells stay behind
Memory cells do not secrete antibodies. Instead, they stay in the body for a long time, carrying the same receptor as the B cell that was selected. Meanwhile, the plasma cells die off and the antibodies are broken down, so the antibody level falls. The antibodies go, but the memory cells stay.
1:52The secondary response
Now the same pathogen comes back. This time memory cells are already there, and they recognise the antigen. They divide and differentiate into plasma cells very quickly, so there is a much shorter lag. Because there are many more cells that respond, more plasma cells are made.
So more antibodies are made, faster, and the concentration reaches a much higher peak. The first time, the body had to find the right B cell and build up a clone from scratch. The second time, a large army of memory cells is waiting. Compare the two peaks. The second is taller, it starts sooner, and it rises more steeply. This is the secondary response.
2:34Why no symptoms the second time
And the result for the person is that the pathogen is often destroyed before it makes them ill. That is why you usually only get some diseases once, at least where the antigen stays the same.
3:23Examiner trap and recap
The trap. Memory cells do not secrete antibodies. They become plasma cells, and plasma cells secrete the antibodies.
Recap. Plasma cells make the antibodies. Memory cells make the second response faster and larger.
Next time, we use exactly this idea to explain how vaccines work, and what herd immunity means.
Watch first
This lesson builds on:
- B Cells, Clonal Selection and Antibodies · 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-045.