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’s nothing short of a medical marvel — an unsung hero in keeping us healthy, and a potential gold mine for new treatments.
“It’s really this masterpiece of biological engineering in my mind,” said Katharina Ribbeck, a professor of biological engineering at the Massachusetts Institute of Technology whose lab is dedicated to the study of mucus. The sticky substance is essential to many of the body’s functions. It can help us swallow and digest our 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/U.N.C. 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’s not just in your nose — it’s everywhere.
More than 95 percent water by weight, mucus lines all of the wet surfaces inside the body — not just your nose, but also your 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, Texas, 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, Dr. Dickey said.
Mucus might be gross, but it’s really good for you.
Mucus keeps the cells on our internal surfaces wet so they can do their jobs. It covers the olfactory cells in our nose and taste buds on our tongues, enabling us to smell and to taste food well, Dr. Ribbeck said. It prevents our 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 fertilize an egg.
Mucins are “workhorses” of our immune response, Dr. Ribbeck said.
They bind to pollutants, bacteria and viruses that enter our bodies and trap them so we 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 said, 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 makeup of your mucus can affect the composition of your microbiome, said Rebecca Carrier, a professor of chemical engineering at Northeastern University 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, Dr. Carrier said.
Certain threats send mucus into overdrive.
When a virus moves from your nose 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 can’t push it out, said Dr. Judith Voynow, a pediatric pulmonologist at the Children’s Hospital of Richmond at V.C.U. 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 can’t 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, like ulcerative colitis and Crohn’s disease, mucus also becomes abnormal, Dr. 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, Dr. Carrier said, 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.
How do they study the sticky substance?
Dr. Carrier, an unabashed mucus enthusiast, was more than happy to explain. One way, she said, is to collect samples from pig intestines, scrape the mucus off, pipette it into vials and freeze it for experiments. Or scientists can collect human saliva, extract mucins and recreate a simpler version of the gel found in the human body. A third option involves placing human tissues on a chip to more closely mimic an actual gut.
Researchers can then add bacteria and observe how they move through the mucus and how mucins interact with them.
Dr. 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.
Dr. Ribbeck said scientists are working to develop mucin-based treatments, including using artificial mucus to replace lost or defective mucus. They’re also exploring the idea of administering engineered mucins with specific sugars to help the body defend 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.
