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Defense Mechanisms of the Respiratory System

ByRebecca Dezube, MD, MHS, Johns Hopkins University
Reviewed/Revised May 2025
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The average person who is moderately active takes about 22,000 breaths per day, which is a constant exposure to the surrounding air. Sometimes, air contains potentially harmful particles and gases. Larger particles, such as dust and soot, mold, fungi, bacteria, and viruses deposit on airway and alveolar surfaces, where the respiratory system has defense mechanisms to clean and protect itself. Only extremely small particles, less than 3 to 5 microns (3/1000 to 5/1000 of a millimeter, or about 2/10000 of an inch) in diameter, penetrate to the deep lung.

Cilia, tiny muscular, hair-like projections on the cells that line the airway, are one of the respiratory system's defense mechanisms. Cilia propel a liquid layer of mucus that covers the airways.

The mucus layer traps pathogens (potentially infectious microorganisms) and other particles, preventing them from reaching the lungs.

Cilia beat more than 1,000 times a minute, moving the mucus that lines the trachea upwards about 0.5 to 1 centimeter per minute (0.197 to 0.4 inch per minute). Pathogens and particles that are trapped on the mucus layer are coughed out or moved to the mouth and swallowed.

Alveolar macrophages, a type of white blood cell on the surface of alveoli, are another defense mechanism for the lungs. Because of the requirements of gas exchange, alveoli are not protected by mucus and cilia—mucus is too thick and would slow movement of oxygen and carbon dioxide. Instead, alveolar macrophages seek out deposited particles, bind to them, ingest them, kill any that are living, and digest them. When the lungs are exposed to serious threats, additional white blood cells in the circulation, especially neutrophils, can be recruited to help ingest and kill pathogens. For example, when the person inhales a great deal of dust or is fighting a respiratory infection, more macrophages are produced and neutrophils are recruited.

(See also Overview of the Respiratory System.)

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