Explain how the human body's non-specific defence systems and the immune system work together to protect the body when a bacterial pathogen enters through a cut in the skin.

WJEC A-Level Biology (Wales) — 3.5 Human impact on the environment · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

The skin and blood clotting form a mechanical barrier that prevents the bacteria from entering the body.
White blood cells recognise the pathogen through receptors that bind to antigens on its surface.
Phagocytes such as macrophages engulf the bacteria and digest it using enzymes (phagocytosis).
Lymphocytes produce antibodies that are complementary to the antigen; the antibodies bind to the bacteria and neutralise it.
After the infection, memory cells remain, so if the same pathogen enters again it is destroyed more quickly.

Examiner tips

  • Use the term ‘mechanical barrier’ for skin/clotting; mention ‘receptors’ for pathogen detection; include ‘phagocytosis’ and ‘enzymes’ for digestion; state ‘antibodies’ and ‘neutralise’ for lymphocytes; finish with ‘memory cells’ for rapid response.
  • Follow the order of the mark scheme – each point should be a separate sentence or bullet.
  • Use UK spelling (e.g. ‘recognise’, ‘neutralise’).

Common mistakes

  • Mixing up non‑specific and specific defence terms; e.g. calling antibodies part of the non‑specific system.
  • Omitting the role of memory cells or the specific action of antibodies.
  • Using the wrong verb tense or pluralising ‘antigen’ incorrectly.

Mark scheme (5 marks)

  1. The skin / blood clotting acts as a mechanical barrier to prevent pathogens entering (non-specific defence)
  2. White blood cells detect the pathogen using receptors that identify antigens on the pathogen's surface
  3. Phagocytes engulf and digest the pathogen (phagocytosis) using enzymes
  4. Lymphocytes produce antibodies that are specific to / complementary to the antigen and bind to it to neutralise the pathogen
  5. Memory cells remain after the infection so that if the same pathogen enters the body again, it can be destroyed more quickly

Key terms in this question

non-specific defence system · immune system

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