ACHOO! Ughh

One of my earliest childhood memories is of a big old chunk of snot.

It was stuck in my nose, and I had to blow really hard to get it out. When it did, I was fascinated to see that it was actually a sort of squishy, rubbery cube, and a sickly, pea-soup green.

This being flu season, there are probably a lot of you out there looking at your hands right now and wondering why your snot is so green.

Jennifer Bawden, science director at the Telus World of Science in Edmonton and an immunologist by training, says most people produce about a litre of snot – or mucus – a day. You reabsorb some of it, swallow more of it, and sneeze or flake off the rest of it.

The slimy gunk comes from mucosal membrane cells that line the eyes and lungs as well as the digestive and urinary tract, Bawden says. You’ve got about 400 square metres of mucosal membranes in your body – enough to cover a basketball court.

Mucus goes by many names. When it comes from the nose, we call it snot. It goes by phlegm in the lungs and sputum once it comes out of our mouth.

The substance itself is composed mainly of water and a substance called mucin, which is a protein with a bunch of sugar molecules branching off, Bawden says.

“They kind of look like a bottle-brush,” she says of the proteins. That brush-like structure makes mucus sticky when wet and crusty when dry.

Medical researchers say mucus serves several functions in our body. First, it keeps various tissues moist so they don’t dry out and crack, which would let infections in.

Second, its brush-like structure acts as a sticky trap to capture pollen and dust before it clogs up our lungs.

Third, it contains many substances that crush bacteria and viruses before they get into our bodies, Bawden says.

Some of these are lysozymes – bacteria-eating enzymes commonly found in specialized white blood cells called lysosomes.

Also present is lactoferrin, a protein that binds to free iron in the environment so that bacteria can’t use it to reproduce.

And you’ve got immunoglobulin A (or IgA), which is an antibody that binds to bacteria to stop them from attaching to cells, Bawden continues – a vital defence against salmonella and polio.

Katharina Ribbeck of the Massachusetts Institute of Technology recently discovered that mucin stops bacteria from clumping together to form nigh-invulnerable structures called bio-films. Mucin coats could therefore protect medical devices from contamination, while mucin toothpaste could boost the immune system.

Researchers with the Icahn School of Medicine learned in 2013 that mucus in the large intestine not only physically stops bacteria from invading the body, but also suppresses the inflammation those bacteria can cause.

Mucosal membranes kick into overdrive when you have a cold as they try and flush out the infection, Bawden says – that’s why you get a runny nose.

“When it gets to be that gross, greenish-yellow colour, that’s the point at which your body’s immune system has kicked in,” she explains.

“What you’re seeing is white blood cells in your snot.”

Mucus can also turn red if you have a bleeding nose, yellow if you’re in a high-pollen area, or grey or black if there’s a lot of air pollution.

Researchers are of mixed minds when it comes to eating nose snot. While some say that you shouldn’t because it contains pathogens, University of Saskatchewan biochemist Scott Napper has theorized that doing so might strengthen the immune system by teaching it what to defend against – the fact that snot tastes sugary could be a signal for us to eat it.

Bawden says she’s not sure why anyone would want to eat their snot, but advised anyone who did to make sure they were eating their own snot first.

If you want to lessen your chance of green snot on your hands, get a flu shot and reduce your risk of infection.

Alternatively, use a tissue.