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Meet the real culprits responsible for your nasty cold or flu symptoms

Colds and flu are caused by viruses, but their symptoms — from runny nose and sore throat to chills and green mucus — are actually caused by a handful of chemicals in our own immune systems.

Key points for flu explainer

Key points

  • All symptoms of cold and flu are by-products of inflammation.
  • Sore throat and sneezes are caused by chemicals setting off nerve signals.
  • Blocked noses are literally that — blocked by swollen veins.
  • The green stuff isn't bacteria, it's white blood cells.

From the moment you breathe in a cold or flu virus, it starts invading and destroying cells, one at a time. But that cell destruction isn't what makes you feel sick — it isn't even the source of all that liquid dripping from your nose.

It's the inflammation triggered by chemical messengers in your immune system that's behind the sneezing, dripping, aching mess you've become.

Inflammation is all about getting your immune cells — T cells and other white blood cells — to where the virus is, so they can physically destroy and mop up the virus particles. That involves signalling the location of the viral infection, and then swelling up the blood vessels nearby and making them leaky so the white blood cells can get out into the tissue more easily.

And all that coordination is managed by a trio of chemical messenger molecules — cytokines, bradykinin and prostaglandins.

Cytokines are the first respondents — as soon as a virus is detected, patrolling white blood cells churn out cytokines to attract more white blood cells to the area. These small proteins also signal other cells to release bradykinin and prostaglandins, which work together to re-engineer the blood vessels.

But those changes aren't the only things these molecules do. Between them, cytokines, bradykinin and prostaglandins are behind every symptom you need to justify a few days in bed.

Sore throat, sneezing, blocked nose and cough

A scratchy throat is often the first sign of a cold or flu coming on, and like all early symptoms of a cold or flu it's a by-product of inflammation.

The bradykinin and prostaglandin molecules that run the inflammation process also act on pain-sensing nerves in the throat, causing a tickle and then outright pain.

These chemical mediators are also responsible for the sneezing in colds and flu.

Sneezing is triggered by nerves in the nasal tissue and the top part of the throat. Bradykinin and prostaglandins stimulate these sensory nerves, which then signal the brain's sneeze centre to get ah-chooing. (In allergies, another molecule, called histamine, signals sneezing).

Coughing is triggered by sensory nerves in the airway, below the throat. Some scientists think that as the inflammation spreads past the nose and back of the throat, bradykinin acting on these nerve endings could be behind the dry cough that often appears in the early days of a cold.

But the cough story is complicated and can't be pinned on a single factor just yet.

Bradykinin and prostaglandins don't just act on nerves. By causing blood vessels to dilate — the keystone of inflammation — these molecules are also behind your soon-to-be blocked nose. As the veins in the nasal tissue swell in size they physically block the airways in your nose.

A similar bit of blocked plumbing can close off your tear ducts, leading to watery eyes.

Headache, fever, chills and cough

While bradykinin and prostaglandins are behind the nose/throat action, the general aches and pains, chills, fever and loss of appetite are down to cytokines.

Your body temperature is controlled by an internal thermostat in the hypothalamus of your brain. By interacting with nerve endings connected to this temperature control centre, cytokines are likely to cause chills and shivers, as well as the fever that accompanies flu.

Likewise, cytokines are probably behind the headaches. While the mechanism of their action isn't fully understood, giving healthy volunteers a dose of some types of cytokine can induce everything from a headache to fever.

The green stuff

As for the colour of the stuff that you blow out your nose or cough up, this is about the only symptom of a cold or flu that is caused by a different chemical altogether.

While many of us are stuck on the idea that green means bacterial, there's actually no evidence that this is true.

The colour is caused by a green-coloured protein, called myeloperoxidase, present in some infection-fighting white blood cells.

In the early stages of inflammation the white blood cell numbers are low, and the discharge is clear or white. But as the number of white blood cells increases, so does the amount of green myeloperoxidase, and the colour becomes yellowish and eventually green.

Thanks to Professor Ronald Eccles, former director of the Common Cold Centre at Cardiff University.

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