[♪ INTRO].

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].