Describe how the human body defends itself against pathogens before they enter the body, and explain what happens when a pathogen does manage to enter the bloodstream.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Non‑specific barriers:
1. Physical – skin, hair in nose, mucous membranes and blood clotting prevent entry.
2. Chemical – mucus, tears, stomach acid destroy or inhibit pathogens.
If a pathogen enters the bloodstream:
• Phagocytes (macrophages, neutrophils) engulf and digest it (phagocytosis).
• Lymphocytes (B‑cells) produce antibodies that bind to antigens on the pathogen, neutralising it.
• Some lymphocytes become memory cells, giving a faster response on re‑infection.
1. Physical – skin, hair in nose, mucous membranes and blood clotting prevent entry.
2. Chemical – mucus, tears, stomach acid destroy or inhibit pathogens.
If a pathogen enters the bloodstream:
• Phagocytes (macrophages, neutrophils) engulf and digest it (phagocytosis).
• Lymphocytes (B‑cells) produce antibodies that bind to antigens on the pathogen, neutralising it.
• Some lymphocytes become memory cells, giving a faster response on re‑infection.
Examiner tips
- Use the exact terms: phagocytosis, antibodies, memory cells. Show the sequence: barrier → phagocyte → antibody → memory. Keep the answer concise – 5 marks, so one sentence per point.
Common mistakes
- Mixing up specific and non‑specific barriers. Failing to mention memory cells or antibody binding. Using vague terms like "immune system" without detail.
Mark scheme (5 marks)
- Named non-specific physical/mechanical barrier (e.g. skin, hairs in nose, blood clotting)
- Named non-specific chemical barrier (e.g. mucus, stomach acid, 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 again, a faster immune response occurs
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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