Lesson 3 of 5 / Immune recognition and memory
Clonal selection and immune memory
Why is a second response usually faster for the same antigen?
In this lesson: Explain B/T-cell selection, expansion, plasma cells and primary/secondary responses.
About 7 min
The key ideaAntigen 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.
Is this true? "Memory means antibodies can recognise every new antigen equally well."
Memory is specific to previously encountered antigenic features, with possible cross-reactivity rather than universal recognition.