Explain how B lymphocytes and T lymphocytes differ in their roles during the adaptive immune response.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
B lymphocytes recognise free, extracellular antigens and, when activated, differentiate into plasma cells that secrete specific antibodies into the bloodstream.
T lymphocytes recognise antigens presented on the surface of cells via MHC molecules. Helper T cells activate other immune cells, including B cells and cytotoxic T cells, while cytotoxic T cells directly kill infected or abnormal cells by inducing apoptosis or cell lysis.
Thus, B cells produce antibodies against extracellular antigens, whereas T cells coordinate the response and kill infected cells without producing antibodies.
T lymphocytes recognise antigens presented on the surface of cells via MHC molecules. Helper T cells activate other immune cells, including B cells and cytotoxic T cells, while cytotoxic T cells directly kill infected or abnormal cells by inducing apoptosis or cell lysis.
Thus, B cells produce antibodies against extracellular antigens, whereas T cells coordinate the response and kill infected cells without producing antibodies.
Examiner tips
- Use the word ‘recognise’ and ‘presented on MHC’ to show understanding of antigen presentation.
- Mention both helper and cytotoxic T cells to cover the full range of T‑cell functions.
- Show the distinction that B cells secrete antibodies, T cells do not.
- Keep the answer concise – 4–5 sentences is enough for full marks.
Common mistakes
- Confusing B cells with plasma cells – remember plasma cells are the antibody‑secreting form of B cells.
- Saying T cells produce antibodies – they do not; they activate other cells.
- Lumping all T cells together without distinguishing helper and cytotoxic functions.
Mark scheme (4 marks)
- B lymphocytes produce / secrete antibodies (as plasma cells)
- T lymphocytes do not produce antibodies; instead, T helper cells activate other immune cells (B cells and cytotoxic T cells) / coordinate the immune response
- Cytotoxic T lymphocytes directly kill infected / abnormal cells (e.g. virus-infected cells or tumour cells) by inducing apoptosis / cell lysis
- B lymphocytes respond to antigens that are free / extracellular, whereas T lymphocytes respond to antigens presented on cell surfaces (via MHC / major histocompatibility complex proteins)
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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