Tuberculosis (TB) is a communicable disease caused by the bacterium Mycobacterium tuberculosis. It is spread when an infected person coughs or sneezes, releasing droplets into the air. Explain how TB spreads from one person to another and describe two ways the human body attempts to prevent the pathogen from causing infection once it has been inhaled.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Tuberculosis (TB) remains one of the leading infectious diseases worldwide. It is caused by the bacterium Mycobacterium tuberculosis and primarily affects the lungs. The disease is most prevalent in overcrowded living conditions where people are in close proximity for extended periods.
Model answer (5 marks)
TB spreads when an infected person coughs or sneezes, releasing droplets or aerosols containing Mycobacterium tuberculosis into the air.
In overcrowded or close‑contact settings the droplets travel easily between people, increasing the risk of transmission.
Once inhaled the body uses two main defence mechanisms:
1. The mucus produced by goblet cells in the airways traps the bacteria, acting as a physical barrier.
2. Cilia on the ciliated epithelial cells beat to move the mucus (and trapped bacteria) out of the lungs toward the throat, where it is swallowed and destroyed.
If any bacteria reach the lung tissue, phagocytic white blood cells engulf and destroy them by phagocytosis.
In overcrowded or close‑contact settings the droplets travel easily between people, increasing the risk of transmission.
Once inhaled the body uses two main defence mechanisms:
1. The mucus produced by goblet cells in the airways traps the bacteria, acting as a physical barrier.
2. Cilia on the ciliated epithelial cells beat to move the mucus (and trapped bacteria) out of the lungs toward the throat, where it is swallowed and destroyed.
If any bacteria reach the lung tissue, phagocytic white blood cells engulf and destroy them by phagocytosis.
Examiner tips
- Use the exact terms ‘droplets’, ‘aerosols’, ‘goblet cells’, ‘cilia’, and ‘phagocytosis’ to gain full marks.
- Explain the role of overcrowding to justify the spread mechanism.
- Show the two defence mechanisms in separate points for clarity.
Common mistakes
- Confusing ‘aerosols’ with ‘droplets’ or omitting the airborne aspect.
- Failing to mention both mucus and cilia as separate defence mechanisms.
- Using vague terms like ‘immune response’ without specifying phagocytosis.
Mark scheme (5 marks)
- TB is spread through the air via droplets / aerosols released when an infected person coughs or sneezes (droplet transmission / airborne transmission)
- Overcrowding / close contact increases the risk of transmission because droplets travel easily between people in close proximity
- Mucus in the airways (produced by goblet cells) traps the bacteria / pathogens, acting as a physical barrier
- Cilia (hair-like structures on ciliated epithelial cells) waft / move the mucus (and trapped bacteria) away from the lungs towards the throat where it is swallowed
- White blood cells / phagocytes engulf and destroy any bacteria that reach the lungs / tissues by phagocytosis
Key terms in this question
communicable disease · pathogen
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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