Mucus: It’s snot what you think

NEW YORK – To many people, mucus is nothing more than a gross goo, something to be tossed away in a tissue immediately after clearing your throat or blowing your nose.

To scientists, it is nothing short of a medical marvel – an unsung hero in keeping people healthy and a potential gold mine for new treatments.

“It’s really this masterpiece of biological engineering in my mind,” said Massachusetts Institute of Technology professor of biological engineering Katharina Ribbeck, whose laboratory is dedicated to the study of mucus.

The sticky substance is essential to many of the body’s functions. It can help people swallow and digest food, disarm harmful bacteria and even help build a healthy microbiome.

“Mucus has got a bad rap,” said Dr Richard Boucher, a pulmonologist and director of the Marsico Lung Institute/UNC Cystic Fibrosis Center at the University of North Carolina at Chapel Hill. “People think it’s something you should spit out and get rid of, but it’s an amazing substance.”

It is not just in your nose – it is everywhere

More than 95 per cent water by weight, mucus lines all the wet surfaces inside the body – not just the nose, but also the eyes, ears, throat, airways, lungs, gut and urogenital tract.

Its key building blocks are mucins, which are long proteins decorated with complex sugars that make them look like bottlebrushes.

These sugars help attract water, Dr Boucher said, and the mucins form a gel-like mesh that can cover a large surface area like the nasal cavity, keeping it wet while also controlling the passage of pollutants, bacteria, viruses and drugs through it.

“It’s designed to protect you from the outside world,” Dr Boucher said.

The mesh structure is what gives mucus its “visco-elasticity”, said Dr Burton Dickey, a pulmonary and critical care physician at MD Anderson Cancer Center in Houston, who has studied mucus properties. That makes it thick enough to stay in place on top of cell surfaces, but stretchy enough to be moved out of the airways, he added.

Mucus might be gross, but it is really good for you

Mucus keeps the cells on internal surfaces wet so they can do their jobs. It covers the olfactory cells in the nose and taste buds on the tongues, enabling you to smell and taste food well, Prof Ribbeck said. It prevents one’s eyes and throat from drying out and getting irritated.

In the gut, mucus stops the stomach from eating itself by walling off stomach acid from the stomach lining. It also helps move food through the intestines. And in the cervix, mucus enables sperm to swim through and fertilise an egg.

Mucins are the workhorses of your immune response, Prof Ribbeck said.

They bind to pollutants, bacteria and viruses that enter the bodies and trap them, so that your body can mechanically expel them. “In the next 10 minutes you’re going to inhale a thousand bacteria, and you’d like to clear them without ever knowing about it,” Dr Boucher said.

You do this, he added, with the help of tiny structures on the airways called cilia, which beat 10 times a second – “like little oars” – and sweep mucus up to the throat, where it is swallowed and digested.

When you blink, you essentially “squeegee off mucus” from your eyes into ducts that lead down to your throat, he said. And when your stomach and intestinal muscles squeeze food along, they propel mucus through the gut.

Just as mucins bind to bad bacteria, the sugars on them can recruit good microbes too, by acting as a source of food.

Specific sugars will feed specific bacteria, meaning that the make-up of your mucus can affect the composition of your microbiome, said Northeastern University professor of chemical engineering Rebecca Carrier, who studies intestinal mucus.

Mucins can also affect the function of immune cells, many of which live in your gut, and either stimulate or tamp down the body’s inflammatory response, she added.

Certain threats send mucus into overdrive

When a virus moves from your nose and down your respiratory tract, your body tries to defend itself by making more mucus.

Sometimes it makes too much, and the cilia on the airways cannot push it out, said Dr Judith Voynow, a paediatric pulmonologist at the Children’s Hospital of Richmond at VCU in Virginia. The result is a stuffy nose, phlegmy throat or blocked airway. The feeling of built-up mucus can trigger a cough, which forces it out.

In certain chronic diseases of the airways, like asthma, chronic obstructive pulmonary disease or cystic fibrosis, mucus becomes too concentrated, which makes it overly sticky and hard to expel.

In these cases, Dr Voynow said, mucus cannot clear out bacteria and viruses. The mucus eventually forms plugs that can block the airways and lead to chronic infections, inflammation and damage to the airways.

In inflammatory diseases of the gut, such as ulcerative colitis and Crohn’s disease, mucus also becomes abnormal, Prof Carrier said. In a healthy colon, there are usually two layers of mucus – a dense inner layer devoid of bacteria and a looser outer layer where bacteria live.

But in studies of these diseases, she added, scientists have detected bacteria throughout both layers, which leaves underlying tissues vulnerable to damage.

It holds the potential to help treat disease

Scientists are working to better understand how mucus works, what makes it go awry and how it might be harnessed to treat health problems.

Prof Ribbeck’s research has shown that the sugars on mucins can effectively switch off mechanisms that the bacteria involved in strep throat or cholera, for example, or fungus in a yeast infection, use to go from innocuous to harmful.

“The microbes proliferate just fine, but don’t switch on their weaponry – presumably because they have enough food,” she said.

Prof Ribbeck said scientists are working to develop mucin-based treatments, including using artificial mucus to replace lost or defective mucus.

They are also exploring the idea of administering engineered mucins with specific sugars to help the body defend itself against infections without creating resistance to drugs, as can happen with antibiotics.

A substance with that kind of potential is a far cry from mere snot. NYTIMES