Explain how clonal selection leads to the production of plasma cells following exposure to a pathogen.

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)

Antigens on the pathogen bind to complementary antibodies on the surface of a B lymphocyte. Only the B cells with a receptor that matches that antigen are selected and activated. The activated B cell proliferates by repeated mitosis to form a clone of identical cells. Some of these clonal cells differentiate into plasma cells, which secrete large quantities of specific antibodies.

Examiner tips

  • Use the term ‘B lymphocyte’ and ‘antigen–receptor complementarity’.
  • Show the sequence: binding → selection → proliferation → differentiation into plasma cells.
  • Mention that plasma cells are the antibody‑secreting cells.
  • Keep the answer concise – one sentence per point.

Mark scheme (4 marks)

  1. Antigens on the pathogen bind to complementary / specific antibodies on the surface of a B lymphocyte (B cell)
  2. Only the B cell(s) with a complementary receptor to that specific antigen are selected / activated
  3. The selected B cell undergoes repeated mitosis / proliferates to form a clone of identical cells
  4. Some clonal cells differentiate into plasma cells that secrete large quantities of specific antibodies

Key terms in this question

clonal selection · plasma cell

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