Explain how the human body defends itself against pathogens using both non-specific defences and the immune system.

WJEC A-Level Biology (Wales) — 3.1 Photosynthesis · Explain · 5 marks · View as Markdown

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

The human body has several layers of defence against pathogens. These include non-specific defences that are always present and a specific immune response that is triggered when a pathogen enters the body.

Model answer (5 marks)

1. The skin and mucous membranes act as a physical barrier, preventing many pathogens from entering the body.
2. White blood cells recognise pathogens by detecting antigens on their surface.
3. Lymphocytes release antibodies that are complementary to the antigen; the antibodies bind to the pathogen, neutralising it and causing agglutination.
4. Phagocytes engulf and digest pathogens through phagocytosis, using enzymes to break them down.
5. After an infection, memory cells persist, enabling a faster and stronger antibody response if the same pathogen re‑enters the body.

Examiner tips

  • Use the exact terms: skin, mucous membranes, white blood cells, antigens, lymphocytes, antibodies, phagocytes, memory cells.
  • Show the sequence: barrier → recognition → antibody production → phagocytosis → memory.
  • Keep each point concise to match the 5‑mark allocation.
  • Mention that antibodies are specific to the antigen.

Common mistakes

  • Confusing the role of antibodies with that of phagocytes; they act in different stages.
  • Failing to mention that memory cells enable a quicker response on re‑exposure.
  • Using vague terms like "immune cells" instead of specific types such as lymphocytes or phagocytes.

Mark scheme (5 marks)

  1. Named non-specific defence acting as a barrier (e.g. skin, mucus, stomach acid, hairs in the nose, blood clotting)
  2. White blood cells detect pathogens by identifying antigens on the pathogen's surface
  3. Lymphocytes release antibodies that are specific/complementary to the antigen and bind to it to neutralise the pathogen / antibodies cause agglutination
  4. Phagocytes engulf and digest pathogens (phagocytosis) using enzymes
  5. Memory cells remain after infection so that if the same pathogen is encountered again, antibodies are produced more quickly

Key terms in this question

non-specific defence

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