Explain how vaccination confers protection against a specific pathogen without the individual being exposed to the active disease.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The vaccine contains antigens (or weakened/killed pathogens, attenuated pathogen, or sub‑unit proteins) that do not cause disease but are recognised as foreign. These antigens stimulate an adaptive primary immune response, leading to clonal selection and expansion of specific B and/or T lymphocytes. Memory B cells and/or T cells are produced and persist long‑term. When the individual is later exposed to the real pathogen, these memory cells mount a faster and larger secondary immune response, producing antibodies quickly enough to prevent disease.
Examiner tips
- Use the exact terms: antigens, adaptive immune response, clonal selection, memory cells, secondary response. Show the sequence from vaccine to memory to protection. Keep answer concise and to the point.
Common mistakes
- Confusing innate with adaptive immunity; describing the vaccine as causing disease; omitting the role of memory cells or the speed of the secondary response.
Mark scheme (4 marks)
- The vaccine contains antigens (or weakened/killed pathogens/attenuated pathogen/subunit proteins) that do not cause disease but are recognised as foreign (non-self).
- The antigens stimulate an adaptive (primary) immune response, leading to clonal selection and expansion of specific B and/or T lymphocytes.
- Memory cells (B memory cells and/or T memory cells) are produced and persist long-term in the body.
- Upon later exposure to the real pathogen, memory cells enable a faster and larger secondary immune response, producing antibodies quickly enough to prevent disease.
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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