Explain how clonal selection and clonal expansion lead to the production of plasma cells during an adaptive immune response.

IB DP Biology Standard 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)

An antigen is recognised by a B lymphocyte that carries a receptor complementary to it. Only the B cells with the matching receptor are activated – this is clonal selection. The activated B cells then undergo rapid mitotic division, producing a large clone of identical cells – clonal expansion. Within this clone, some cells differentiate into plasma cells, which secrete large amounts of specific antibodies.

Examiner tips

Use the exact terms ‘antigen’, ‘B lymphocyte’, ‘clonal selection’, ‘clonal expansion’, ‘plasma cells’, and ‘antibodies’.
Show the sequence: antigen → specific B cell → selection → expansion → differentiation into plasma cells.
Mention that plasma cells are the antibody‑secreting effector cells.
Keep the answer concise – 4 marks allow a short paragraph or 4 bullet points.

Common mistakes

Confusing T helper cells with B cells; the question refers to B lymphocytes for plasma cell production.
Omitting the step that only B cells with the complementary receptor are activated (clonal selection).
Failing to state that plasma cells are the differentiated product of the expanded clone.

Mark scheme (4 marks)

An antigen is presented to / recognised by a specific B lymphocyte (or T helper cell) that carries a complementary receptor
Clonal selection is the process by which only the B lymphocyte(s) with the complementary receptor are selected / activated
Clonal expansion is the rapid mitotic division of the selected B lymphocyte, producing a large clone of genetically identical cells
Some of the cloned cells differentiate into plasma cells, which secrete large quantities of specific antibodies

Key terms in this question

clonal selection · clonal expansion · plasma cell