I tend to get a lot of summer colds, in fact I’ve probably got one now. Today at the Society’s Annual Conference, we’ll be learning a lot more about the symptoms of the common cold and what causes them…
“They often have a relatively poor diet, are frequently stressed and usually don’t get enough sleep – students are the perfect volunteers.”
These are the words of Professor Ron Eccles, who is presenting a talk today at the conference, entitled Understanding the symptoms of common cold and flu. Ron works at Cardiff University’s Common Cold Centre, where he works with industry to better develop symptomatic cold treatments. The Centre tests their remedies on volunteers who come in when they first begin to get a cold. As he suggests, the university’s student populace gives them a large pool of people to work with.
Ron’s talk describes the body’s typical reaction to the common cold. However, ‘typical’ is a difficult word to use –some people are beset by colds all year round, while others never seem to get ill. This is all down to an individual’s immune system, which Ron believes research is starting to show “is as unique as our looks or our personality.” Indeed, the immune system of the student volunteers is relatively immature, as they have yet to be exposed to the countless multitude of rhinoviruses (the major cause of the cold) in the environment. The immune system of older people has had time to develop, hence why they get fewer colds.
For many people, a cold starts with a dry, ’scratchy’feeling in an area at the back of the throat known as the nasopharynx. The cells here are the viruses’ point of entry and the immune system starts to take notice. Cells infected with viruses present small amounts of viral proteins on their surface. These molecules are recognised by patrolling immune cells known as neutrophil granulocytes that in turn release interleukins – chemical signals that rouse the rest of the immune system into action.
There are usually a small number of granulocytes found in nasal mucus, but the release of the interleukins causes them to rush in huge numbers from the blood stream into the mucus to fight the infection. This migration causes normally clear mucus to change colour from clear to white, yellow or green. The common misconception is that this colour change is a sign of bacterial infection, when really it’s the immune system doing its job. In fact, the green colour can be attributed to a peroxidase enzyme produced by the neutrophil granulocytes that contains iron, giving it the green tint.
Aside from triggering the body to produce more immune cells to fight the infection, the interleukin messengers travel to the brain and cause the body to do some very strange things indeed…
One of the responses to a cold infection we’re all familiar with is a rise in body temperature. The cold virus replicates best at temperatures lower than that of the body, so raising it can help to slow the infection. However, breathing in cold air keeps the temperature in the nose lower than in the body – around 32°C – ideal for the virus to replicate. To compensate for this, the body causes blood vessels in the nose to swell, which, combined with the mucus being produced, effectively seals the nasal cavity and raises the temperature to slow the virus. The body also alternates between blocking the left and right nostril, meaning that you can always breathe, albeit at a lower level.
Something else that happens when you’re fighting a cold – you feel rotten. Shivering helps to keep your internal temperature high, Professor Eccles suggests, while the ‘chills’ is an evolutionary response that sees your body look for a warmer environment to keep your temperature up. Eccles also explains that during a cold your mood temporarily changes: you become lethargic, lose your appetite and just want to sleep. This is known as ‘sickness behaviour’ and is found throughout the animal kingdom, not just in humans, but in other mammals, insects and even molluscs!
This behaviour is an energy saving response. In humans, suppressing our hunger also suppresses our foraging/hunter-gatherer and exploratory tendencies. Not wanting to eat for a few days isn’t a problem. Those pains you get in your limbs when you’ve a bad cold? That’s your body breaking down the protein in your muscles to use as an energy source. The pain is caused by the action of a molecule called prostaglandin, which sensitises nerves during muscle breakdown. Aspirin inhibits the synthesis of prostaglandins, which explains why it’s found in most cold remedies.
The reason for cold-related lethargy is also down to energy. While you’re awake your brain uses around 25 per cent of the body’s total energy. While you’re asleep, this energy expenditure is greatly reduced, and can be diverted to the immune system.
These symptoms usually last for up to five days, during which time a virus can be spread between people. The body mounts an antibody attack against the cold virus, completely clearing it out in around two weeks – unlike more severe viral infections, rhinovirus does not lie dormant in the body.
While cold viruses cause sickness behaviour, they are so successful because they don’t generate severe sickness. People often travel when they have a cold, infecting their colleagues or fellow students. Also, colds prompt a relatively minor immune response that is not long lived. That means you can catch the same cold twice, unlike other viral diseases like chickenpox.
A question we’ve all wondered when we wake up with a cold is: ‘Should I really go to work today?’ Professor Eccles has been asked this question by companies and government organisations many times. His response?
“If you think you should send your staff home, you’d be better off just shutting your office from October until March, these viruses are so common. That said, they aren’t hugely contagious. Most colds are caught at home, where you’re in close proximity to your family. In the office you tend to have more space. Just remember to wash your hands…”
Image credit: Wikimedia