Snot's not that bad - in fact, it's great.

You are leafing through James Joyce's Ulysses when your eye alights on the famous line 'The snotgreen sea. The scrotum-tightening sea'. Later in the day you are reading Paul Theroux's novel My Other Life when you come across this line: 'I would rather chew snot than stay in this house'. Have you, you wonder, stumbled upon an untapped vein of literary deconstruction? The Significance of Snot and Other Effluvia in the Plays of William Shakespeare. Nasal Discharge as Metaphor in the Fiction of Jane Austen.

That's all very well, but, as noted by Joyce, why is snot green, you hear yourself ask? The green colour comes from immune cells called neutrophils. When you get a cold these neutrophils engulf the bacteria and begin to destroy them using digestive enzymes. Some of these enzymes, such as lactoferrin, are dependent on iron for their activity. The colouration comes from the iron - ferrous iron compounds are green. After a while, snot turns yellow as other cells start moving in and dying.

The yuck factor aside, snot, or 'nasal mucus' as scientists prefer to call it, is an interesting substance and one that may hold the key to a cure for cystic fibrosis and Crohn's disease and even an anti-cancer vaccine. We'd be in big trouble if we could not produce mucus. People who don't produce mucins in the intestine, for example, have constant chronic infection and diarrhoea.

Barrier against Noxious Substances

In the nose, mucus acts as a barrier against germs, dust and other noxious substances. We breathe between 10,000 and 30,000 litres of air each day - which carries pollen, germs and a great deal of other gunk. These get trapped in the mucus surface and destroyed by white cells and enzymes. As medical writer Brad Evenson explains:

The process does not stop there. The air passages are lined with tiny arms called cilia. Each waving 1000-1500 times a minute. These cilia push a carpet of mucus out of the lungs at 20 millimetres a minute - like a rock star being carried along on the upstretched arms of concert fans - until it is swallowed.

As Tasty as Junk Food

Parents who tell their children to get their fingers out of their noses may be doing them a disservice. Immunologists say that it is important to prevent an accumulation of mucus in the functional tubes. And it's not surprising that kids like to dine out on bogies (boogers in the US). Dried nasal discharge can be as tasty as junk food because it has basically the same ingredients - complex sugars, sodium and water. Though we produce - and consume - about a litre of mucus a day, it has proved to be extremely hard to grow in the laboratory.

All is not lost though. Even though the scientists are experiencing difficulties replicating mucus, you can make a passable stringy imitation with corn syrup and gelatine in the microwave. Sadly, the commercial product that came packaged in a huge plastic nose - which you apparently ate by sucking through the nostrils - is no longer available.

Bronchitis and Cystic Fibrosis

Of course, you can have too much of a good thing. When you get a cold, mucus goes into overdrive in an attempt to shed the virus and you drip. No sooner have you blown your nose than the mucus-producing cells lining the nasal cavity extract more mucus from your blood. Experiments show that on a peak day, the average person produces 14 grams of drippings. Too much mucus can also lead to bronchitis when it obstructs the airways. A mucus defect is the cause of cystic fibrosis. Children born with the disease produce mucus that is too thick and sticky, making it difficult for them to breathe. About 95 percent of them die of respiratory failure.

In 1989, however, geneticist Lap-Chee Tsui was able to isolate the single gene that caused the defect in mucus production. Gene therapy may eventually be able to restore the mucus of cystic fibrosis sufferers to normal viscosity.

A Link between Mucus and Cancer

Scientists have also discovered a link between mucus production and cancer. Researchers have found that tumours secrete mucins that protect them (the tumours) from attack by the body's own immune system. Based on that knowledge a Canadian company, International Medical Innovations, has developed a screening test for cancer. Called 'LungAlert', the test requires patients to give a sputum sample, which is then analysed to determine if a specific mucin is present. The test detected lung cancers in 87 percent of study cases. Many were in the early stages when tumours can be removed surgically.

A colon cancer test has also been developed, and the company believes the approach may work with cervical and prostate cancers. Scientists are hopeful that a vaccine can be developed that kills off the mucin-producing cell in a tumour, allowing the body's white cells to attack and destroy the tumour.

Dig with Pride

It's clear that we have underestimated the role and importance of mucus. Its revolting reputation is unjustified. Instead of being a surreptitious nose-picker, dig out your bogies with pride.

Alternatively, you could follow the example set by the Basotho people of Lesotho in southern Africa who use small metal implements for picking their noses. In a touching display of motherly love, their womenfolk clear their babies' noses of snot by sucking it out.