Why is snot green? Immunologist answers 8-year-old’s burning question (and reveals the grim colour is actually good news for your immune system!)

  • When someone has a cold or infection immune cells kill off the virus or bacteria
  • When the immune cells, which contain a green chemical, die they end up in snot
  • The green colour is a sign your body is successfully fighting an infection 

It is thought to be a sign that a sniffle is something more like a cold or a worse infection, and isn't something you'd be likely to ask your doctor about.

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But having green snot is actually a sign your immune system is working well and you're getting better, according to an expert.

Kim Murphy, an immunology researcher at Monash University in Australia, says the green colour actually comes from immune cells which have died in battle against an infection.

Dr Murphy explained the phenomenon in response to a question from eight-year-old Xavier from Clifton Hill, a suburb of Melbourne.

Xavier asked 'Why does my snot turn green when I have a cold?'

Dr Murphy gave her explanation for The Conversation

Snot has a green colour when you are sick because immune cells, which contain a green chemical, come out in mucus after they have died while fighting an infection
Snot has a green colour when you are sick because immune cells, which contain a green chemical, come out in mucus after they have died while fighting an infection

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).   

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.  

Immune cells destroy bugs and damaged cells 

WHY YOU GET A RUNNY NOSE

Excess mucus signals there is inflammation, most likely due to an allergy or infection.

Inflammation irritates the nasal lining and dilates the blood vessels there – leading to a runny nose.

What usually happens is the cilia (tiny hair-like structures inside the nose) sweep mucus away towards the back of the throat and we swallow it without realising.

But in cold weather, these cells act more slowly, or even become paralysed, which is why your nose runs.

However clear, runny mucus is a good sign because it means your nasal passages aren't fighting off a cold.

It also means the water, antibodies, enzymes and protein and dissolved salts that make up mucus, can get on with their job – keeping your nasal passages moist. 

Source: Dr Sarah Brewer, GP 

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When we get sick our immune system needs to get rid of the virus and make us better. 

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.

When neutrophils – a type of immune cell – die they end up in snot 

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).

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.

Chemical in immune cells is green 

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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