Trevor Norton, of Hampden, asks :-

What are the constituents of phlegm and snot? Why can at times one produce great quantities of this material, and other times can't?

Julian Crane, a respiratory research specialist at the University of Otago Medical School (Wellington), responded

Phlegm and snot are pretty much the same thing, the former from the chest the latter from the nose. Mucous of course is also produced in the gut. It is made up of many substances but mainly water and mucins (mainly glycoproteins) which give mucous its slippery characteristics and is produced in specialised glands lining the airway and gut. Its function is mainly to protect the airway from the nose to the depths of the lung from inhaled foreign material and to provide a first defence against bacteria and viruses both physically and because it contains various antimicrobial proteins. Within the airways there exists on the surface beneath the mucous lining cells with tiny cilial projections that 'beat' in a co-ordinated fashion to continually waft the mucous up the airway and into the oesophagus taking with it any foreign material and bacteria. In the nose excessive, often more watery, snot can be produced as part of an allergic reaction, often to pollens (hay fever).

Mucous is produced in larger quantity following any insult like inhaling anything noxious or most often in response to infection. In the case of bacterial infection this gives rise to a colour change to yellow or green and this colour change is produced by large numbers of white cells from the blood stream that enter the airway to engulf and destroy bacteria. Long term smokers produce excessive mucous and is a feature of the chronic bronchitis that they often develop.

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