What is snot? What is its purpose? How septic is the nose compared to the mouth? Why is snot yellow or green?
Snot is your body's best defense mechanism, a sticky moat of protection against invading bacteria, viruses, and fungi. When it comes to where your body is open to the outside world, snot (more properly, mucus) provides a barrier against these alien invaders.
Mucus, chemically, is quite fascinating. Sugar chains are attached to a protein backbone in mucus cells, with the contraption released out into the open. These glycoprotein molecules rapidly and aggressively suck up water until they are plump, slick, and slimy. To an invader, this is a nightmare to navigate: tangled chains of protein and sugar, with every nook and cranny crammed with water molecules. (Boogers are when these chains become ever more tangled, finally resulting in a rubbery ball of partially dried-out snot. Neat!) The body adds antimicrobial enzymes to this mix, which digest the invading organisms as they slowly attempt to chew through this barrier and reach the thin underlying lining of cells.
As the outer layers of snot are eaten or rubbed away, new layers are forming underneath—creating a sort of treadmill of slime for invaders to run on. Hence, during an infection, our bodies tend to make more snot in an attempt to run the invaders out.
Although the surplus of snot is not much fun when we're sick, it's better than the alternative. People with cystic fibrosis have a damaged chloride receptor, preventing them from properly filling their snot with water. Without the nice slick snot, people with the disease are subject to all sorts of terrible infections—particularly in their lungs.
Snot turns colors as the defensive enzymes within ramp up to attack invaders. Many of the attacks involve charging up metal ions—turning them into nastily reactive bombs against the invaders. For example, green snot comes from iron-ramped-up white blood cells.
The human mouth remains the champion of sepsis—containing the most bacteria per unit of any normally functioning body part by far, aside from perhaps the later stretches of the gut. This makes the mucus of the lungs all the more remarkable. Initially, the air entering the lungs is full of pathogens. As the air takes many twists and turns down to the delicate and vulnerable alveoli, one by one the pathogens get stuck in the sticky mucus lining the passages. By the time the air reaches the alveoli, it has been scrubbed; air at the very ends of the lung is sterile—free of bacteria.
All together, the body makes about a liter of snot a day—probably a bit more in the average toddler.
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