When it comes to science, kids come up with some of the best questions, things we grownups wouldn’t normally think to look into. So when my kids came to me last week and asked, “Dad, what does mucus do?”….well, how could I pass that up?
It’s in there somewhere…actually, everywhere
You know mucus when you see it, because when you have a cold, you usually see a lot of it. Mucus is a viscous fluid made by tissues called mucous membranes that line the body’s openings and tubes: the nose, sinuses, mouth, respiratory tract (windpipe and lungs), and gastrointestinal tract — everything from esophagus to anus — as well as the eyes and genitals. And you make a lot. When healthy, you produce upwards of a liter to a liter and a half of mucus every day, most of which just gets swallowed. (Ewww.)
In all the places you find mucus, it forms a protective barrier that keeps the delicate tissues underneath from drying out. But it does more specific things when you look at particular organs:
- Gastrointestinal tract: It’s a lubricant, helping food slide through.
- Stomach: Mucus forms a wall between the lining of the stomach and the acids that digest our food. Without it, the stomach would digest itself.
- Respiratory tract: It’s kind of like a moat, trapping would-be invaders. Bacteria, dust particles, and other things get stuck in the sticky coating and pushed back up toward open air by tiny, hair-like structures called cilia. From there, we sneeze, cough, hork or swallow the little buggers.
So what’s mucus made of, anyway? It’s a mixture of water, salt, dead cells, and mucins — long, heavy glycoproteins (proteins with sugar groups attached to them at various points). The mucins get all tangled up into a big mesh, which gives mucus its almost gel-like consistency and its microbe-trapping abilities.
We’ve all seen how our mucous changes when we get sick, especially the mucus from the nose. When you have a cold, it gets thicker and can start to change color, often appearing yellow or green. Both colors come from white blood cells that march to the mucus to clear out the cold; an enzyme produced by white blood cells called myeloperoxidase is what’s responsible for the greenish hue.
Sometimes the mucus is the product of disease itself, not just the body’s reaction to one. Cystic fibrosis patients harbor a mutation in a gene called CFTR, resulting in extremely thick, viscous mucus. Asthma can also trigger excess mucus.
All the gut’s a stage….
As we learn more about mucus and the cells and other things that reside within it, a picture emerges of mucus as a stage, upon which many plays are acted. For instance:
Scene 1: A border crossing. Setting: the GI tract. This is the only place where we run large amounts of foreign matter (food) through our body on purpose. As a result, the cells and tissues of the GI tract see a lot of stuff.
Here is where mucus plays a really important role. It hosts a unique form of immunity called mucosal immunity. Embedded within the mucus lining of the GI tract are antibodies (in particular, a kind of antibody called IgA), white blood cells (leukocytes), antimicrobial proteins and enzymes such as lysozyme* and defensins, and scads upon scads of bacteria (the gut flora, which we touched on a couple of weeks ago). Together, this mix of immune players decides whether a particular microbe or bit of food is a benign one to be tolerated, or an invader to be repulsed, making the mucosal immune system an attractive target for developing a broad range of vaccines.
Recent research suggests that mucus is also chock-full of bacteria-killing viruses. The viruses, called phages, attach themselves to the sugar portions of mucins and lay in wait. When a bacterium comes floating by, they ambush it, take it over, use it produce millions of baby phages, and kill it. The mucins within it can also prevent disease-causing bacteria from forming clingy biofilms.
Mucus even harbors bacteria that eat it (you can find microbes that will eat just about anything). Akkermansia muciniphila lives in the mucus in your gut and munches mucus. And it’s good that it’s in there, because a recent study found that A. muciniphila influences weight and metabolism.
It’s a scene that’s played out in the mucus of other parts of the body as well, such as eyelids and the linings of the reproductive and respiratory tracts. The players on each stage are slightly different, but the plot is the same: working together, this motley cast does yeoman’s work to protect the body from the environment we live in.
So show some respect for your mucus, and the tiny neighbors that live in it. They’re busy keeping you safe from the bad world outside.
*Lysozyme is an interesting story in and of itself. It was discovered in the 1920s by Alexander Fleming, father of penicillin and, by extension, of the antibiotic age. Lore has it he had a runny nose, and it dripped onto one of his culture plates. When he looked later, he found that the bacteria he was culturing wouldn’t grow in the spot where the drip fell.