Explain how the human body defends itself against pathogens before they enter the body and how the immune system responds once a pathogen has entered the body.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Non‑specific barriers:
1. Physical – skin and mucous membranes (nose hairs, tears) block entry.
2. Chemical – stomach acid, mucus, lysozyme in tears destroy or inhibit microbes.
Once a pathogen breaches these barriers the immune system acts:
3. Phagocytes (macrophages, neutrophils) engulf and digest the invader (phagocytosis).
4. Lymphocytes (B‑cells) recognise antigens, produce specific antibodies that bind and neutralise the pathogen.
5. Some B‑cells become memory cells, remaining after the infection so that a second exposure triggers a faster, stronger response.
1. Physical – skin and mucous membranes (nose hairs, tears) block entry.
2. Chemical – stomach acid, mucus, lysozyme in tears destroy or inhibit microbes.
Once a pathogen breaches these barriers the immune system acts:
3. Phagocytes (macrophages, neutrophils) engulf and digest the invader (phagocytosis).
4. Lymphocytes (B‑cells) recognise antigens, produce specific antibodies that bind and neutralise the pathogen.
5. Some B‑cells become memory cells, remaining after the infection so that a second exposure triggers a faster, stronger response.
Examiner tips
- Use the exact terms: phagocytosis, antibodies, memory cells; include both physical and chemical barriers; keep the answer concise and in the correct order.
- Show the sequence: barrier → phagocytosis → antibody production → memory. 5 points total. 1 point each.
- Use UK spelling (e.g. ‘phagocytosis’).
Common mistakes
- Mixing up specific and non‑specific barriers; forgetting to name both physical and chemical examples.
- Not naming the antibody‑producing lymphocytes or the memory cell function; using vague terms like ‘immune response’ without detail.
Mark scheme (5 marks)
- Named non-specific physical/mechanical barrier (e.g. skin / hairs in the nose / blood clotting sealing a wound)
- Named non-specific chemical barrier (e.g. stomach acid / mucus / tears)
- Phagocytes engulf/ingest and digest pathogens (phagocytosis)
- Lymphocytes produce antibodies that bind to antigens on the pathogen to neutralise it
- Memory cells remain after infection so that if the same pathogen enters the body again, the response is faster/more rapid
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Population size and ecosystems questions
- Explain how memory cells protect the body more effectively when the same pathoge…
- Describe how the human body produces monoclonal antibodies and explain one way i…
- Explain how the human body uses antibodies to destroy a pathogen after it has en…
- Describe the role of lymphocytes and phagocytes in destroying a pathogen that ha…