Mucus is often pictured as a germ trap, and the research adds a twist. Mucins disarm harmful bacteria without killing them, which makes it less likely the bacteria will evolve resistance and spares the helpful bacteria that live on mucus membranes. In a dish, MIT scientists put mucins together with bacteria and found that the mucins stopped the bacteria from sticking and clumping together, which kept them from doing harm; mucus works as part of the immune system, since it blocks germs and harmful particles and moves trapped things out of the body, and yet one scientist says the textbook view is that mucus is a barrier to infection, but that it is not at all clear how it does so.

Start with what mucus is. Snot is mucus, a sticky, slimy stuff that the nose makes, and clear, thin mucus is normal and healthy; everybody makes it. Cells in the mouth, throat, nose and sinuses make it, and the nose and sinuses make about a quart (about 1 liter) every day, although as of November 2026 experts say nobody knows the exact amount; one description says mucus is made of water, surface cells, dead white blood cells, mucin and mineral salts, while another lists water, mucin proteins, defense proteins and salt. In healthy airway mucus, water makes up about 98 percent of the mass and mucins about 0.7 percent, with salts and a few other large molecules making up the rest.

The part that makes mucus mucus is mucin, the main part of mucus, which gives it its gel-like feel. Goblet cells, shaped a bit like a fancy drinking glass, make it; mucin gets its slipperiness from sugars attached to a protein backbone, and sugar chains make up more than 70 percent of the mass of the mucins in airway mucus. The sugars give mucins a brush-like shape, and without them the protein folds into a ball and has less space to hold water; an experiment shows how much the sugars matter. Working with mucus from pig stomachs, MIT scientists stripped the sugars off mucins, and the mucins lost their water-soaking and slippery qualities.

Water is what the brush is for. Mucins are packed dry inside goblet cells, and after release they must soak up more than 100 times their own mass of water to turn into the right kind of gel for the cilia to move. MIT researchers found that mucins bind far more water than other proteins they tested, and in other MIT lab tests the coating mucin formed on a surface was 90 percent water. Set that beside the 98 percent water in healthy airway mucus and the picture is of a material that is mostly water in both measurements. In MIT friction tests, a solution containing mucins was several orders of magnitude slipperier than a solution that held only salts.

The airway keeps its mucus in layers. Airway mucus has two main layers: a sticky gel layer on top, and a watery layer underneath that surrounds the cilia and lets them beat; a thin layer of surfactants sits between the gel and watery layers. A model called gel-on-brush, which describes the mucus gel and the brush-like watery layer under it, accurately predicts how fast mucus moves, based on how strongly each layer pulls in water; the model links the speed of the mucus to the pull of each layer on water.

The lining of the nose has two main kinds of cells, ciliated cells that sweep and secreting cells that make mucus. Each ciliated cell in the nose carries about 100 to 250 cilia, and each cilium is about 5 to 7 micrometers long, roughly a tenth the width of a human hair; the cilia wave back and forth all the time, moving a bit like arms swimming the breaststroke. Each cilium has a power stroke that pushes the mucus toward the mouth and a recovery stroke that brings it back to rest, ready to grab the next bit of mucus; cilia line the airways and push mucus along, like an escalator.

The escalator does steady work. In the nose, cilia clean the nose about five times an hour, which works out to one full cleaning about every twelve minutes; almost all the dust and germs we breathe in get trapped in mucus, and the cilia carry that mucus toward the throat. When we swallow it, it meets stomach acid, which helps destroy the harmful material in it. In the lung, cilia and airflow, including coughing, clear mucus, and how fast mucus moves depends heavily on how concentrated the mucus is. The system can be slowed: a cold virus can slow the cilia in the nose, and the effect can last for several weeks.

The nose is not a sterile place, and that is part of the defense. It has its own community of microbes, called a microbiome; mucus, the nose's microbes and antibodies together form a front line of defense against infections. Nasal mucus carries antibodies and enzymes, so it works as a chemical shield as well as a trap; antibodies and enzymes are not the only guards, since the nose's own microbes and the mucus itself work alongside them. The front line has more than one member; pulling out nose hairs can cause a bad infection, so they are best left alone.

How do scientists measure something this small and this wet? Scientists once tracked nasal cilia by putting a tiny drop of mildly radioactive fluid in the nose and following it with a scanner, a test since replaced by others. A 1977 Mayo Clinic study measured nose airflow every 15 minutes for about 7 hours in 50 people, which is about 28 readings for each person, and found that 72 percent, or 36 of the 50, had a clear nasal cycle. A 2016 study recorded airflow in each nostril for 24 hours in 33 healthy people, a longer window with fewer people; the 24 hours of 2016 are more than three times the 7 hours of 1977, so the later study covered a full day and night.

The nasal cycle is when airflow shifts back and forth between the two nostrils, so one side is more open than the other. At any moment you breathe mostly through one nostril while the other rests. Sources give different lengths for a turn. One description says the busy side switches about every two hours, and another gives a mean length of about two and a half hours; the 2016 study found that the cycle lasted about 2 hours when people were awake and about 4.5 hours in sleep, which is more than twice as long. People usually do not notice any of it, because the total resistance to airflow through the nose stays fairly constant even as the two sides trade places.

What drives the swap, and why does it exist? Researchers say nose airflow differs between nostrils because erectile tissue in the nasal passage swells and shrinks, and that the cycle is related to the body's autonomic arousal. The nasal cycle is controlled by a central regulator in the hypothalamus; the 2016 study found that lying on one side led to more airflow through the nostril on the opposite side, and if someone lies on their right side, gravity makes the lower nostril more congested. No one is sure why the nasal cycle exists, even though total airflow stays almost the same; one idea is that moisture builds up on one side so that it does not get too dry.

Hay fever is one common cause of a runny nose. It is an allergy to pollen, the fine powder plants make. In hay fever, IgE antibodies sit on mast cells, and when enough pollen binds to them, the mast cells release granules of histamine and other chemicals; histamine causes the runny nose, sneezing and swelling. In a study of twelve people with birch-pollen hay fever, those with more histamine in the nose lining during pollen season had stronger nose symptoms; hay fever tends to come back at the same time every year, and in the UK it is usually worse between late March and September.

A cold and an allergy can look alike, but time sorts them out. A runny or blocked nose is a main sign of a cold, and a cold usually gets better on its own in about one to two weeks; a person with a cold stays infectious until the symptoms are gone, and can spread it a few days before they begin. A cold usually gets better in one to two weeks, but hay fever can last weeks or months, and allergy symptoms usually come on suddenly and last as long as a person is exposed to the allergen.

Color gives hints, but it does not decide anything. The color of snot alone does not tell you what is going on inside the body; yellow mucus often shows up at the start of an infection, when white blood cells are working hard, and green mucus shows up when the body fights a more severe or longer-lasting infection, but it is only one clue. Blood can make mucus pink-tinged or streaked with red; once an illness is over, mucus slowly returns to its normal color.

A booger is dried mucus mixed with bits of dust, pollen and dirt, and nose picking has a long formal name, rhinotillexomania. Other primates do it too; even so, picking the nose can make it bleed, and boogers contain lots of germs, so a tissue is a better tool. Blowing your nose into a tissue is a good way to clear out boogers, and a tissue traps germs; if soap and water are not handy, the CDC says to use a hand sanitizer with at least 60 percent alcohol (as of November 2026).

The next chapter may be built rather than found. Scientists are studying how to design fully synthetic mucins that copy the real ones; mucus lines the inside of the nose and also the lungs and the digestive tract, and it coats the stomach in a layer about one millimeter thick, which keeps strong stomach acid from digesting the stomach itself. A material that is about 98 percent water, made slippery by sugar chains, swept along by cilia and able to disarm a bacterium without killing it, is still teaching researchers how a barrier works.

Words to know

• mucins: mucus gel proteins
• surfactants: slippery film makers
• microbiome: community of microbes
• autonomic: automatic body control
• resistance: pushback
• rhinotillexomania: nose picking