Re: Why does the body produce mucus?

Mucus performs more than one function.

Mucus is primarily secreted from cells lining the tracts running through our body that connect to the outside world: the respiratory, gastrointestinal and genitourinary tracts. They occur as well where body surfaces not covered in skin come in contact with the environment, such as the conjunctivae around the eye. In general, we call these mucosal surfaces, referring to the thin barrier layer there (called the mucosa) that lacks the thickened overlying layer of keratin present in skin.

In the gastrointestinal tract and in the female genital tract, mucus appears to act primarily as a lubricant. Saliva contains mucus, as do gastric secretions, secretions of the small intestine, and secretions from the colon. As the gastrointestinal tract moves food through its length, mucus is vital to its normal passage. This is clear in diseases where mucus production is impaired, such as cystic fibrosis, or where salivary secretion is impaired, such as Sjorgen syndrome or as a side effect of medications. Mucus produced by the stomach is also essential to protection of the gastric wall from the strong acid that the stomach produces to sterilize food and break down food particles. Its production in the stomach can be impaired by infections with certain bacteria that are associated with peptic ulcers.

In the respiratory tract and other mucosal surfaces, mucus has an immune function as a barrier between tissue and the outside world. It traps dust and foreign particles (such as viruses) as air enters the respiratory tract. It also serves as a medium where the immune cells and proteins on these surfaces can interact with potential invaders, such as viruses and bacteria. The lining of our respiratory tract also has cells specialized to move the mucus along and out, so that particles and invaders do not remain in the respiratory tract. Impairment of mucus production here, as we see in the abnormally thick, dehydrated mucus of people with cystic fibrosis, is a primary reason that cystic fibrosis is assoicated with recurrent and chronic pneumonias. The lungs of people with cystic fibrosis become colonized and infected repeatedly with bacteria that rarely cause pneumonia in people without cystic fibrosis, primarily because of the problems with producing normal mucus. Mucus also provides protection from foreign invaders in the uterine cervix and contributes to barrier protection in the urethra.

Our bodies tend to produce more mucus in the presence of invading organisms. We see extra mucus in our respiratory tracts with colds and pneumonia; we see it in our stool with intestinal infections, and the genitourinary tracts produce extra mucus when they become infected as well. We can safely presume, I think, that mucus represents to the layperson the sign of infection in another. One might be "grossed out" by mucus because it represents something one might like to avoid, or something one didn't like about one's own body's last infection.

Mucus production can be impaired by certain medications, some of them present in cold remedies. Their effects are temporary, though reducing mucus production often leads to thicker mucus. Thicker mucus is more difficult to eliminate from places where poor clearance of mucus can lead to a focal infection, such as the middle ear cavity or the sinuses. It's important to weigh that risk with the benefit that the medications provide in relieving mucus production, such as obstruction of the nose that impairs sleep or (in newborns) normal breathing. For children in particular, cold remedies have a poor record of benefit and provide many side effects, some of them dangerous.

The FDA recommended just last week that all children under 6 years of age not receive over-the-counter cold rememdies because they are proven neither effective nor safe. The Washington Post article (with links to sources supporting the recommendation including the American Academy of Pediatrics, the American College of Chest Physicians, the Journal of the American Medical Association, the Cochrane Collaboration, and the Centers for Disease Control and Prevention) link is www.washingtonpost.com/wp-dyn/content/article/2007/10/25/AR2007102502619.html . None of the over-the-counter cold remedies are recommended for use in children under 6 months of age.

Tim Nicholls, MD
Berkeley, CA