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  • We all have that friend or coworker who insists that they never get colds

  • because their immune system is just so strong.

  • But research shows they might want to hold off on that boast.

  • People with more robust immune responses

  • may actually be more likely to show symptoms of a cold.

  • And that's because cold symptoms aren't caused by cold viruses,

  • but by your body trying to show those viruses the door.

  • The common cold is usually caused by a class of viruses called rhinoviruses.

  • Unlike many other viruses, like the flu,

  • rhinoviruses don't damage or destroy the cells of your airway, though some do fall away.

  • Instead, the symptoms you experience, like a sore throat and stuffy nose,

  • are actually caused by your own immune system responding to the virus.

  • Not everyone who is infected by a cold virus actually shows symptoms.

  • In fact, about 25% of rhinovirus infections have no symptoms at all.

  • And it's not because these people have built up immunity to those particular viruses.

  • A 2003 study looked at healthy infants who visited the doctor

  • either for a routine checkup or for a respiratory infection.

  • They found that of the infants who seemed healthy and were just there for a check-up,

  • 20% of them showed evidence for a rhinovirus infection, but no symptoms.

  • And those babies hadn't had much time to build up immunity,

  • to colds, or anything at all, really.

  • In people who do have cold symptoms,

  • the severity of those symptoms often correlates with markers of inflammation.

  • These markers include cytokines: molecules secreted by immune cells

  • that are involved in cell communication and interaction.

  • A cytokine called IL-8 has been shown to increase

  • in people with viral inflammation of the nose.

  • IL-8 is involved in recruiting white blood cells to infections.

  • This in turn heightens your inflammatory response,

  • resulting in pain and a stuffy, runny nose.

  • And a number of studies have shown that there are greater levels of IL-8

  • in people with more severe cold symptoms.

  • In one 1998 study, volunteers were inoculated with a cold via virus-filled nose drops.

  • Whether or not they developed an infection was measured

  • by flushing their noses with saline in the days after the inoculation,

  • then checking to see if that fluid contained viruses that could infect other cells in a lab.

  • People who did get an infection either showed no symptoms and no IL-8 increase,

  • or they felt sick and their levels of IL-8 went up.

  • In other studies, severity of symptoms similarly didn't seem to correspond with

  • how many copies of the virus are present in your body.

  • In fact, in a 1994 study, washing healthy subjects' noses with IL-8

  • was enough to induce cold-like symptoms; no virus needed!

  • This makes it seem like the bigger your immune response,

  • the worse your symptoms will be.

  • Taken together, the evidence suggests that people with stronger

  • or more reactive immune systems may actually experience worse cold symptoms.

  • So the next time you find yourself sniffling and sneezing,

  • surrounded by tissues, blame your immune system.

  • But don't be too hard on it.

  • You probably want your body to respond to colds with a bit of zeal.

  • It's a sign that your immune system will take invaders seriously,

  • and therefore, might just help protect you better

  • against things way more dangerous than the common cold.

  • Thanks for asking, and thanks to our patrons for helping us bring you the answer.

  • If you want to help support SciShow and join our awesome community of patrons,

  • check out patreon.com/scishow.

  • [♪ OUTRO]

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