Explain how the human body defends itself against pathogens before they enter the body and after they have entered the body.

Eduqas A-Level Biology — 3.4 Population size and ecosystems · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Before entry:
1. Skin and hair act as a physical barrier, preventing pathogens from reaching tissues.
2. Chemical barriers such as tears, saliva and stomach acid destroy or inhibit microbes.
After entry:
3. Phagocytes (e.g. macrophages, neutrophils) engulf and digest pathogens by phagocytosis.
4. Lymphocytes release antibodies that bind to specific antigens on the pathogen, neutralising it.
5. Memory cells persist after the infection, enabling a faster, stronger response if the same pathogen re‑enters.

Examiner tips

  • Use the exact terms: skin, hair, tears, stomach acid, phagocytes, lymphocytes, antibodies, memory cells. Show the sequence: barrier → phagocytosis → antibody production → memory. Keep each point brief but complete to hit each mark.

Common mistakes

  • Mixing up specific and non‑specific defenses; e.g., calling antibodies a non‑specific barrier. Omitting the memory cell point or not linking it to faster response.

Mark scheme (5 marks)

  1. Named non-specific physical/mechanical barrier (e.g. skin / hairs in nose / blood clotting sealing wounds)
  2. Named non-specific chemical barrier (e.g. stomach acid / mucus / tears)
  3. Phagocytes engulf / ingest and digest pathogens (phagocytosis) once inside the body
  4. Lymphocytes produce antibodies that bind to specific antigens on the pathogen to neutralise it
  5. Memory cells remain after infection so that if the same pathogen enters the body again, it can be destroyed more quickly

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