Trevor Norton of Hampden asks :-

What is ‘phlegm’ and ‘snot’.

Jim Faed, a pathologist at Otago University's Medical School, responded.

It is called nasal secretions in medical descriptions. All of the moist surfaces of the body produced secretions to keep the thin layer of cells covered with water and to prevent drying and injury. In the case of the nose the secretions come from several types of gland cells in the lining of the nose. The secretions are mostly water together with mucus that gives a slippery and sticky feel to the fluid. If the mucus is present in large amounts it will become very thick and plug-like.

Nose secretions are produced by small pouch like glands that are lined by specialised cells that produce either water and mucus or water and enzyme solutions plus antibacterial substances. The rate at which the secretions are produced varies and can be rapidly increased by nerve signals and local chemical and physical irritation effects.

The secretions are poured out onto the surface where they mix with more mucus produced by a special cell called a ‘goblet’ cell. The old microscopists were very descriptive in the names they applied to tissues in the body and these cells resemble a goblet with the upper part containing a large globular end that is specialised to produce small packets of quite thick sticky mucus. Chemically, mucus is made up of sugar molecules joined together in special sequences.

The secretions are normally carried on a one-way trip to the back of the nose and throat where they are swallowed. This movement is achieved by other lining cells in the nose that have fine projections called cilia that beat in unison like the paddles of a canoe and drive the mucus in one direction. We are normally completely unaware of this process until the amount of mucus and other secretions is increased by an infection, allergy or other irritation.

Mucus in the nose has several functions. It is designed to trap particles that are breathed in. This is achieved because it is wet and sticky and because of the shape of the nose passages that contain ‘turbinate’ bones which create turbulence in the air flow. Another important function is to humidify air before it enters the lungs. As the secretions contain traces of antibodies and other antibacterial substances, mucus in the nose is well designed to provide a ‘moving blanket’ cover that helps to prevent infection and ensures that any organisms are moved on and swallowed. This ensures they meet acid in the stomach and are destroyed.

Most of the time we are completely unaware of the mucus secretions in our nose. During infection or allergic reactions we become very aware of the secretions because they increase enormously and some of the secretions will start to drain forwards. Much of this change occurs because inflammation is present. Inflammation is a defence response against infection and other tissue injuries. If an infection occurs, the secretions will change in colour from clear to yellow or brownish material. Large numbers of inflammatory cells are present in the mucus and mild bleeding may be present.

If inflammation becomes chronic- perhaps due to repeated infections, allergies or other problems, the amount of mucus produced will increase and the mucus becomes thicker and may appear white. The change occurs because the size of the glands and the number of goblet cells increases. After repeated excessive stimulation the glands do not go back to normal after episodes of infection and inflammation settle. As a result the nose may persistently produce increased amounts of thick sticky mucus. This mucus may dry out to a hard crust-like texture if it sits in the front of the nose while air moves past from breathing. The crusted material is just dried out mucus.