Mucociliary clearance: the first-line defence of the conducting airways
Mucociliary clearance is the built-in (innate) defence in which cilia, tiny hair-like structures, beat to move sticky mucus and its trapped microbes up towards the throat.
Remember
Mucus traps inhaled microbes and cilia move it upwards, which makes mucociliary clearance the continuous first-line defence of the conducting airways (the trachea and bronchi that carry air to the lungs). Cigarette smoke and cystic fibrosis impair it, so chest infections become more likely.
Memory aid
The escalator is a moving staircase, with mucus as the step and the cilia as the motor
The mucus is the step that carries the germs, and the cilia are the motor that drives it up and out.
The airways are lined by ciliated cells, which carry cilia (microscopic hair-like structures), and by goblet cells, which make mucus, a sticky layer. Microbes and particles that are breathed in stick to the mucus. The cilia beat together and move it upwards towards the throat, where it is swallowed or coughed up.
This partnership of mucus and cilia is called the mucociliary escalator, and the process it carries out is mucociliary clearance. It works continuously and does not depend on the body having met the microbe before, so it is an innate defence: one that is built in and acts straight away. It is regarded as the first line of defence in the conducting airways (the trachea and bronchi that carry air to the lungs), and it protects the lower respiratory tract (the airways below the voice box and the air sacs) from infection.
Alveolar macrophages (cells that engulf microbes, working in the air sacs) are a later line of defence, acting on microbes that have already got past the mucociliary barrier. Antibodies made after a previous exposure are part of the adaptive immune system, which is specific and slower to respond. Coughing and sneezing are further expulsion (pushing-out) mechanisms that support the escalator.
Anything that damages the escalator lets microbes linger and makes chest infections more likely. Cigarette smoke damages the cilia and alters the mucus. In cystic fibrosis, a fault in the CFTR protein, which helps control the volume and thickness of the airway surface liquid, leaves the mucus thick and sticky, so the cilia cannot move it effectively. Widened airways, as in bronchiectasis, also compromise these defences.
Key facts
- Mechanism
- mucus traps inhaled microbes and particles, and beating cilia carry it up to the throat to be swallowed or coughed up.
- Where it works
- in the conducting airways, such as the trachea and bronchi that carry air to the lungs.
- Innate
- it is built in, acts straight away and does not depend on having met the microbe before.
- Coughing
- a reflex, and a back-up to continuous mucociliary clearance.
- Smoking
- cigarette smoke damages the cilia and alters the mucus, so clearance works poorly and infections are more likely.
- Cystic fibrosis
- thick, sticky mucus and ineffective ciliary beating mean germs are not cleared, so chest infections recur.
Exam traps
- Alveolar macrophage phagocytosis (engulfing of microbes by cells in the air sacs) is not the first-line defence of the conducting airways, because these cells act after microbes have passed the mucociliary barrier.
- The visceral pleural mesothelium, the covering on the outside of the lungs, cannot filter inhaled particles from inspired air (air that is breathed in), because it does not line the conducting airways.
- The cough reflex is not the characteristic defence that smoking impairs, because it is an intermittent back-up to continuous mucociliary clearance, while cigarette smoke damages the cilia and alters the mucus.
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