Protist Diseases and Human Defence Systems

This lesson covers the protist disease malaria (the only protist disease named on the AQA specification) and provides a detailed look at the body's non-specific defence systems. Understanding how the body prevents pathogen entry is essential before studying the specific immune response in the next lesson.

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What Are Protists?

Protists are eukaryotic organisms — they have cells with a nucleus and membrane-bound organelles. Most protists are free-living and harmless, but some are parasites that cause serious diseases.

Key facts about pathogenic protists:

• They are eukaryotes (unlike bacteria, which are prokaryotes)
• Many pathogenic protists are parasites — they live on or inside a host organism and cause harm
• Some are transmitted by vectors — organisms that carry the pathogen from one host to another without being affected themselves
• The most important protist disease for GCSE Biology is malaria

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Malaria

Malaria is a life-threatening disease caused by a protist parasite of the genus Plasmodium. It is one of the most significant infectious diseases globally, killing over 600,000 people per year, mostly in sub-Saharan Africa.

Feature	Detail
Pathogen	Plasmodium species (a protist parasite) — several species exist, with P. falciparum being the most deadly
Vector	Female Anopheles mosquito
Transmission	The mosquito feeds on an infected person's blood, picking up the Plasmodium parasite; when it feeds on another person, it injects the parasite in its saliva
Symptoms	Recurring episodes of fever, sweating, chills, headaches, muscle pain, vomiting
Serious effects	Can cause organ failure, severe anaemia, cerebral malaria (affecting the brain), and death
Treatment	Antimalarial drugs (e.g. chloroquine, artemisinin-based therapies); some strains are drug-resistant
Prevention	Mosquito nets, insecticides, draining stagnant water, insect repellent

The Malaria Life Cycle

The malaria parasite has a complex life cycle that involves both the mosquito and the human host:

graph TD
    A[Female Anopheles mosquito bites infected human] --> B[Mosquito picks up Plasmodium parasites]
    B --> C[Parasites develop inside the mosquito]
    C --> D[Mosquito bites another human]
    D --> E[Plasmodium injected into human bloodstream]
    E --> F[Parasites travel to the liver and multiply]
    F --> G[Parasites enter red blood cells]
    G --> H[Parasites reproduce inside red blood cells]
    H --> I[Red blood cells burst, releasing more parasites]
    I --> J[Recurring fever and chills as cells burst in waves]
    J --> K[Mosquito feeds on infected human - cycle repeats]
    K --> A

Why Malaria Is Difficult to Control

Several factors make malaria extremely challenging to eradicate:

Challenge	Explanation
No effective vaccine yet	The parasite has a complex life cycle and can evade the immune system
Drug resistance	Some Plasmodium strains have developed resistance to antimalarial drugs
Insecticide resistance	Some mosquito populations have evolved resistance to insecticides
Poverty	The disease is most common in the poorest regions, where prevention is hardest
Climate and geography	Warm, wet climates provide ideal breeding conditions for mosquitoes
Asymptomatic carriers	Some people carry the parasite without showing symptoms, unknowingly spreading it

Exam Tip: Malaria is spread by a vector (the mosquito). The mosquito is NOT the pathogen — the pathogen is the Plasmodium protist. Many students lose marks by saying "malaria is caused by mosquitoes" — it is caused by Plasmodium and spread by mosquitoes.

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Prevention Strategies for Malaria

Because malaria is spread by mosquitoes, prevention focuses on reducing contact between humans and mosquitoes:

Strategy	How It Works
Insecticide-treated bed nets	Physical barrier that also kills mosquitoes on contact; used while sleeping
Indoor residual spraying	Spraying insecticide on walls and ceilings inside houses where mosquitoes rest
Draining stagnant water	Removes mosquito breeding sites (mosquitoes lay eggs in still water)
Insect repellent	Chemical applied to skin or clothing to deter mosquitoes from biting
Antimalarial drugs (prophylaxis)	Travellers to malaria zones can take drugs to prevent infection
Biological control	Introducing fish or bacteria that eat mosquito larvae into water sources

Exam Tip: If asked to evaluate malaria prevention strategies, consider both effectiveness and practicality. Bed nets are cheap and effective but must be used every night. Draining stagnant water is effective but impractical in areas with lots of natural water bodies.

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The Body's Non-Specific Defence Systems — Detailed Study

Before the immune system targets specific pathogens, the body has several lines of non-specific defence that work against all pathogens:

First Line of Defence — Physical and Chemical Barriers

Defence	Location	Mechanism
Skin	Entire body	Continuous physical barrier of tough, dead cells; oil (sebum) has antimicrobial properties
Mucous membranes	Nose, mouth, airways, gut	Produce sticky mucus that traps pathogens
Cilia	Trachea, bronchi	Tiny hair-like structures that beat rhythmically, sweeping mucus and trapped pathogens up to the throat
Stomach acid (HCl)	Stomach	pH of approximately 2 — kills most bacteria and other pathogens swallowed with food
Lysozyme	Tears, saliva, nasal secretions	An enzyme that breaks down bacterial cell walls, killing bacteria
Blood clotting	Skin wounds	Platelets form a clot that seals the wound, preventing pathogen entry

How Cilia and Mucus Work Together

The airways are lined with two types of cells that work together:

1. Goblet cells produce sticky mucus that traps dust, particles, and pathogens
2. Ciliated epithelial cells have tiny cilia on their surface that beat in coordinated waves
3. The beating cilia push the mucus upwards towards the throat
4. The mucus (and everything trapped in it) is swallowed into the stomach where acid destroys the pathogens

This is sometimes called the mucociliary escalator — think of it as a conveyor belt carrying unwanted material out of the lungs.

Exam Tip: The question "Describe how the body prevents pathogens from entering through the respiratory system" frequently appears. Your answer should mention: (1) mucus traps pathogens, (2) cilia sweep mucus upwards, (3) mucus is swallowed, (4) stomach acid kills pathogens. This four-step sequence scores full marks.

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