Describe how the human body defends itself against pathogens. Include both non-specific defences and the role of white blood cells in your answer.

WJEC A-Level Biology (Wales) — 3.3 Microbiology · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

Non‑specific defences:
1. Mechanical barriers – e.g. skin and mucous membranes.
2. Chemical barriers – e.g. stomach acid, lysozyme in tears.
White blood cells:
3. Phagocytes (macrophages, neutrophils) engulf and digest pathogens by phagocytosis.
4. Lymphocytes (B‑cells) recognise antigens on the pathogen and produce specific antibodies.
5. Antibodies bind the antigen, neutralising the pathogen; memory B‑cells remain for a faster response on re‑exposure.

Examiner tips

  • Use the exact terms: phagocytosis, B‑cells, antibodies, memory cells.
  • Show the sequence: barrier → phagocyte → antibody → memory.
  • Keep each point brief but complete – 5 marks total.
  • Mention specificity of antibodies for each antigen.

Common mistakes

  • Confusing T‑cells with antibody production.
  • Omitting the role of memory cells or describing them as ‘long‑lasting’ without linking to faster response.
  • Using vague terms like ‘immune system’ instead of specific components.

Mark scheme (5 marks)

  1. Named non-specific mechanical or chemical barrier
  2. Phagocytes engulf/ingest and digest pathogens (phagocytosis)
  3. Lymphocytes produce antibodies in response to antigens on the pathogen
  4. Each antibody is specific to one antigen and binds to it to neutralise the pathogen
  5. Memory cells remain after infection so a second exposure produces a faster immune response

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