Malaria is a communicable disease caused by a protist called Plasmodium. It is transmitted to humans through the bite of an infected Anopheles mosquito. Explain how the body's defences may prevent Plasmodium from causing infection, and describe how the spread of malaria between individuals can be reduced.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 5.2 Pathogens and infection · Explain · 5 marks · View as Markdown

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

Malaria kills hundreds of thousands of people globally each year. The Plasmodium protist enters the bloodstream when an infected mosquito feeds on a human host. Once inside the body, Plasmodium invades red blood cells and liver cells.

Model answer (5 marks)

The skin is the first line of defence; it prevents Plasmodium from entering unless a mosquito bite breaches it.
If the parasite reaches the bloodstream, phagocytic white blood cells recognise and engulf it, destroying it by phagocytosis.
B‑lymphocytes recognise Plasmodium antigens and produce specific antibodies; these antibodies opsonise the parasite and activate complement, leading to its destruction.
Transmission can be reduced by limiting contact with the mosquito vector – e.g. using insecticide‑treated bed nets or applying mosquito repellent.
Eliminating or draining standing water removes breeding sites, lowering mosquito numbers and thus reducing malaria spread.

Examiner tips

  • Use the exact terms – skin, phagocytes, B‑lymphocytes, antibodies, bed nets, standing water.
  • Show the sequence of defences (physical, cellular, humoral).
  • Mention vector control as a separate point.
  • Keep each point concise to fit the 5 marks.

Common mistakes

  • Confusing the skin with the immune system as a single defence.
  • Failing to name the specific immune cells (phagocytes, B‑lymphocytes).
  • Omitting vector control measures such as bed nets or draining water.

Mark scheme (5 marks)

  1. The skin acts as a physical barrier preventing Plasmodium from entering the body unless the skin is broken (e.g. by a mosquito bite).
  2. If Plasmodium enters the blood, white blood cells (phagocytes) can engulf and destroy it by phagocytosis.
  3. B-lymphocytes produce antibodies specific to the antigens on the surface of Plasmodium, which trigger an immune response to destroy it.
  4. Reducing contact with the mosquito vector prevents transmission, for example by using insecticide-treated bed nets or mosquito repellent.
  5. Draining or reducing standing water (mosquito breeding sites) limits the number of mosquitoes and so reduces transmission of malaria.

Key terms in this question

communicable disease

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