Lesson 3 of 5 / Immune recognition and memory
Clonal selection and immune memory
Why is a second response usually faster for the same antigen?
I can explain B/T-cell selection, expansion, plasma cells and primary/secondary responses.
I can explain B, T, antigen-presenting and memory cells in primary/secondary responses.Syllabus
Syllabus 9477, Extension A(b). Explain B, T, antigen-presenting and memory cells in primary/secondary responses.
About 7 min
Make a guess. You are not marked.
Before you ever meet a new pathogen, do you already have B cells able to recognise it?
Show the answer
A few matching cells already exist.
The repertoire is built before exposure. The antigen selects the matching cells, which then multiply.
The key idea
Antigen selects matching pre-existing lymphocyte receptors; expansion and memory change the speed and scale of later responses.
From recognition to response
Which existing clone expands?
Specificity exists before this exposure. A matching receptor and the required activation signals allow that clone to respond.
Explanation
A diverse repertoire of lymphocytes exists before exposure. An antigen activates appropriate matching cells in a suitable signalling context: clonal selection. These selected cells divide to form many related descendants, termed clonal expansion. The antigen does not act as a mould that directly reshapes every antibody.
Activated B-cell descendants can become plasma cells, which secrete antibodies, or memory B cells. T-cell responses also generate effector and memory populations. Some long-lived plasma cells maintain antibody production, while memory cells can respond when the antigen reappears.
A primary response has a lag while matching cells activate and expand. A secondary response to the same antigen often has a shorter lag, larger antibody output and improved average affinity because memory and prior selection alter the responding population. The exact timing and magnitude depend on antigen and conditions.
Interpret a graph using the time of each exposure, antibody concentration, peak height and decline. An exposure to a different unrelated antigen can still produce a primary response even while a strong secondary response occurs to the original antigen. Memory is specific, not universal protection against every pathogen.
Step by step
- 1
Mark antigen exposures
Distinguish first and repeat exposure.
- 2
Compare lag and magnitude
Use actual data points rather than an assumed threshold.
- 3
Explain the cellular cause
Memory-cell abundance and differentiation support the response.
Worked example
Work through the evidence
At day 30, antigen A is given again and new antigen B is given for the first time. Predict the response comparison.
One way to explain it
A can trigger a faster, larger secondary response; B still requires a primary response from its own matching naive cells.
Why this answer works
- Memory is antigen-specific.
- A previous response does not make every future antigen familiar.
Watch out for this
A student says: "Memory means antibodies can recognise every new antigen equally well." What is wrong with this?
Show the answer
Memory is specific to previously encountered antigenic features, with possible cross-reactivity rather than universal recognition.
Check your understanding
Which order is correct?
Show the answer
Matching cell activation -> clonal expansion -> effector/memory descendants
Selection and proliferation generate a focused response.
Exam-style questions
Check your understanding
After a second exposure to antigen P, antibody to P rises within 3 days. At the same time, a first exposure to antigen Q gives antibody to Q only after 10 days. Which conclusion is best supported?
Show the answer
Memory cells for P but not Q were present.
A fast response needs memory from an earlier exposure. Q has never been met, so it gets a slower primary response.
Quick recall
Why is a second response usually faster for the same antigen?