Curious Kids: Why does my snot turn green when I have a cold?

This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky!

Why does my snot turn green when I have a cold? – Xavier, age 8, Clifton Hill, Victoria.

That’s a great question Xavier! The first thing for us to think about is what is snot? When we are healthy, snot is just what scientists call mucus.

Mucus is a gel that lines our nose, our intestines, and even our lungs. It’s very sticky and slippery. This is because mucus is designed to help keep the germs and bugs out of our body. Any bugs that try and get in, should just get stuck in this mucus and then blown out or swallowed (where they would mostly be destroyed by our powerful stomach acids).

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Sometimes this doesn’t work and the germs infect our body. A cold is caused by a virus, and these like to get inside of our cells and make us sick. When we get sick our immune system needs to get rid of the virus and make us better.

Our immune system

Our immune system is made up of lots of different parts. One part is a special cell called a “neutrophil”. Neutrophils are a type of cell called a “phagocyte”. Phagocyte means a cell that eats things.

Neutrophils like to eat bugs, or our own cells that are damaged by infections like viruses that cause a cold. When we have a cold, neutrophils are one of the cells that help us get better.

These neutrophils work very quickly, but they don’t live for very long. Once they die we need to get rid of them, and they end up in our snot.

Neutrophils have different ways of helping us get better. They can eat bugs, they can send out nets and catch bugs, or they can send out chemicals to kill bugs. All of these processes use a special chemical called MPO (that stands for myeloperoxidase but don’t worry, most scientists just call it MPO).

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MPO is a chemical that makes a type of bleach. Just like you might use bleach when you are cleaning your home, this bleach kills infections. Neutrophils release MPO to kill any germs that it has eaten, or sends it out with its nets, or as one of the chemicals that it releases to kill any bugs.

MPO contains a green colour. And because the dead neutrophils end up in our snot, the MPO in the neutrophils makes our snot look green.

Lots of people think green snot means you are really sick, or that you need antibiotics to treat your infection. But this is not true. Green snot is actually a sign that our immune system is working and that we are getting better.

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Kim Murphy does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.