Question: Why do we get alot of mucus build up when we are sick or have a cold

Andrew Devitt answered on 19 Jun 2013:

Mucus is great stuff. OK so it bungs you up but it helps you to fight infection.

We’ve got some mucus all the time. And it contains lots of anti-microbial things in it. These help to kill invading organisms and protect the cells in your airways. You make more when you’re ill to help clear the infection. And all that snot you make, catches bugs and if you swallow it….. it goes to the stomach and the bugs are killed in the acid.

But it isn’t only made in your airways. It is in your digestive tract (it helps lubricate the food passing through) and in the reproductive system where it again protects against infection (and changes throughout the menstrual cycle).

So it is all good!……. almost. If you’ve got cystic fibrosis, you will have horrible thick mucus that is difficult to shift and that’s a real problem. If you’ve got hay fever, you are probably sick of being bunged up (and this is all because your immune system is reacting to pollen that isn’t really dangerous). Sometimes the mucus can trap bacteria and not kill them so they will grow in the warm moist environment and give you nasty infections like sinusitis.

Most important though…. why is it yellow and green? Well that’s all from the white blood cells that are fighting the infection. One of them (a neutrophil) makes an enzyme called MPO (myeloperoxidase) that it provides the lovely colour.