Trace a primary immune response
Explain the sequence of macrophage, T-lymphocyte and B-lymphocyte actions in a primary immune response.
Section 11.1 · The immune system
AS Level · Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; human subject-teacher review and supervised practical evidence remain separate.
Before you begin
- Pathogens carry antigens, and specific receptors on lymphocytes can recognise complementary antigen shapes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which cell presents processed pathogen antigen to a T-helper cell?
What you will learn
- Describe antigen presentation and activation of T-helper cells
- Explain B-cell clonal expansion, plasma-cell antibody production and T-killer action
| Day | First exposure | Later matching exposure |
|---|---|---|
| Day 0 | 0 | 0 |
| Day 2 | 0 | 5 |
| Day 4 | 2 | 12 |
| Day 6 | 5 | 11 |
| Day 8 | 4 | 9 |
| Day 10 | 3 | 7 |
WHO background: how vaccines work. The plotted values and practice records are invented teaching examples.
Build the explanation
A macrophage engulfs a pathogen, processes its antigens and presents an antigen fragment at its surface. A T-helper cell with a complementary receptor recognises the presented antigen and becomes activated. A B-lymphocyte with a complementary receptor binds the same antigen; signals from activated T-helper cells stimulate it to divide by mitosis. Some cloned B-cells become plasma cells that secrete specific antibodies; others become memory cells. T-killer cells can recognise infected body cells displaying pathogen antigen and destroy those cells.
Apply it carefully
Keep the cells and targets distinct: macrophages present antigen; T-helper cells coordinate activation; B-lymphocytes clone and form plasma cells; plasma cells secrete antibodies; T-killer cells destroy infected host cells. This is the primary response to first exposure and takes time to build. Do not say a macrophage turns into a plasma cell or that antibodies directly reproduce the pathogen.
Pause and explain
What is the main role of a plasma cell?
Worked example
Which cell secretes antibodies after a matching B-lymphocyte is activated?
Show the worked solution
- The activated B-cell divides.
- Some cloned B-cells differentiate into plasma cells.
Answer A plasma cell, formed from an activated B-lymphocyte, secretes antibodies specific to the antigen.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Put these primary-response events in order: plasma cells secrete antibody; macrophage presents antigen; matching T-helper cell is activated; matching B-cell clones.
Give me a hint
Start with antigen presentation before lymphocyte activation.
Compare my reasoning
- Macrophage processes and presents antigen; matching T-helper cell activates.
- Matching B-cell clones and differentiates; plasma cells secrete specific antibody.
A macrophage processes and presents pathogen antigen → a complementary T-helper cell recognises it and activates → a matching B-lymphocyte is stimulated to clone → some clones form plasma cells, which secrete specific antibody.
Look for these in your work
- Events correctly ordered
- T-helper and B-cell roles distinct
- Plasma-cell product correct
A virus-infected body cell displays a pathogen antigen. Which lymphocyte can destroy it, and how does this differ from antibody action?
Give me a hint
Distinguish a host-cell target from a free antigen target.
Compare my reasoning
- A matching T-killer cell can recognise and destroy the infected cell.
- Antibodies bind specific antigens but do not themselves perform T-killer-cell destruction.
A matching T-killer cell can recognise the antigen displayed by the infected body cell and destroy that cell. Antibodies bind specific antigens; this is distinct from T-killer action on infected host cells.
Look for these in your work
- T-killer role stated
- Infected cell target stated
- Antibody role distinguished
A student places plasma cells before B-cell cloning and says macrophages secrete the antibody. Correct both errors.
Give me a hint
Track the lineage from B-cell activation to plasma cells.
Compare my reasoning
- A matching B-lymphocyte clones after activation.
- Some cloned B-cells differentiate into plasma cells, which secrete antibody.
After activation, a matching B-lymphocyte undergoes clonal expansion. Some cloned cells differentiate into plasma cells, and these plasma cells secrete the specific antibody. Macrophages process/present antigen and can phagocytose pathogens; they are not the antibody-secreting cells.
Look for these in your work
- Clonal expansion ordered first
- Plasma-cell lineage correct
- Macrophage role corrected
Common mistakes
- Swapping macrophage, T-helper, B-cell and plasma-cell roles
- Claiming antibodies destroy infected host cells in place of T-killer cells
- Leaving out clonal expansion or antigen specificity
What can you explain now?
Which cell presents processed pathogen antigen to a T-helper cell?
Compare with the explanation
A macrophage
Macrophages can display processed antigens at their surface.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How do macrophages, T-helper cells, B-lymphocytes, plasma cells and T-killer cells contribute to the primary response?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.