What makes mucus green?

Chris - We've all had those really nasty colds where the stuff you're coughing up - you can almost chew it because it's so thick and horrible - and it's a lovely vivid green colour. I actually had a patient who had a very bad pneumonia and I said, "Are you coughing anything up?" And she said, "Oh no" and then preceded to produce this huge blob of literally bright green stuff in front of me and she really had quite a bad pneumonia.

This is actually your body making this stuff. Mucus, in health, is colourless and it's produced by little cells called goblet cells which are in the epithelium - the layer which lines your airways - and this is rich in protein called mucin. When they secrete this substance over the surface of your airways, it attracts water from the surroundings and swells up, and becomes much more voluminous - maybe 600 times. So it swells up as it gets more water into it and it's very sticky. Its job is to trap micro-organisms, pathogens, bits of dust, and debris, that kind of thing, so it's a sort of cleaning thing.

But when you get an infection, which can be viral or bacterial, the infection can damage the cells that line the airways; this damage is intentional on the part of the pathogens. In the case of viruses, because they want to grow in the cells; in the course of growing in the cells, it kills them. In the case of the bacteria, the bacteria kill cells because if they kill a cell then all of the goodies inside the cell, the raw materials, can be liberated and the bacteria scavenge them and use them themselves.

But in the course of doing that, they create quite intense acute inflammation and this releases various inflammatory mediators which attract the immune system to the area, including a class of cell called a neutrophil. These neutrophils have something called a respiratory burst. What that means is they produce enzymes which produce free radicals of oxygen and these free radicals of oxygen destroy the bacteria. But in the process, they can also kill the white blood cell. These myeloperoxidase enzymes - that make this respiratory burst - contain iron as a cofactor, and it's the compounds of this iron, which are present in various oxidation states, that give the mucus its bright green colour.

So, healthy mucus isn't coloured, but when there's an infection going on and there are lots of white blood cells there which kill themselves with this respiratory burst, then the mucus gets this lovely vivid green colour. Yum!