Explain how memory cells contribute to a secondary immune response that is faster and more effective than the primary immune response.

IB DP Biology Higher Level (2023 syllabus) — C3.2 Defence against disease · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Memory cells are long‑lived lymphocytes that remain after the primary response.
On re‑exposure to the same antigen they undergo rapid clonal expansion.
The expanded cells quickly differentiate into plasma cells or effector T cells, producing a larger and faster antibody or cell‑mediated response.
Thus the pathogen is eliminated before symptoms develop, giving a faster and more effective secondary response.

Examiner tips

Use the word ‘memory cells’ and specify B or T cells. Show the sequence: persistence → rapid proliferation → rapid differentiation → higher antibody titres. Mention the outcome: earlier, higher response and symptom prevention.

Common mistakes

Confusing primary with secondary response. Forgetting to state that memory cells are long‑lived. Omitting the rapid proliferation step or the higher antibody concentration.

Mark scheme (4 marks)

Memory cells are long-lived lymphocytes (B or T memory cells) that persist after the primary immune response.
On re-exposure to the same antigen, memory cells rapidly proliferate by clonal expansion.
Rapid differentiation of memory cells into plasma cells / effector T cells produces a larger and faster antibody response / cell-mediated response than during the primary response.
The pathogen is eliminated before symptoms develop / the secondary response is more effective because antibody levels peak sooner and at a higher concentration.

Key terms in this question

memory cells · secondary immune response