THE SCIENCE BEHIND SNOT

Why mucus thickens, changes colour and what it reveals about your immune response

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When you're sick you'll often produce more phlegm, and might notice it's thicker or a different colour: white, green, yellow or maybe even brown.

What can this phlegm - also called mucus, snot, sputum, catarrh and booger - really tell us about our health?

Here's what to look for, and when to see a doctor.

What is phlegm?

We all produce mucus, even when healthy. Mucus is a barrier to the outside world that helps protect the organs in our bodies.

It's produced by special cells within the epithelium. This is a tissue that lines the organs, cavities and surfaces in your body, including your eyes, mouth, digestive system and respiratory system (nasal passages and lungs).

Cells in this epithelial lining constantly secrete mucus. It acts as:

Why do we produce more mucus when we're sick?

The epithelium is one of the body's first lines of defence when we have an infection. When these cells detect a pathogen - whether bacteria or a virus - they produce more mucus to strengthen the body's physical and immune barriers.