What is the biological purpose of having a runny nose during a cold?

Note, this is a different question than "What causes a runny nose" and other similar questions. Those question are about the cause, what starts the nose running. This question is about why? What is the body trying to do by making the nose run.

For example a cough, I'm guessing, is to help remove or loosen gunk in the lungs. Tears seem to have a point of cleaning the eyes. Diarrhea seems to have a point of clearing the bowels. Fevers have a point of killing viruses. Vomiting seems to have a point of getting rid of bad things in the stomach. Sweating seems to have a point of cooling the body down.

During a cold though, while reasons like inflammation etc cause a runny nose, what's the point? Why did we evolve to have runny noses during colds? What's the body trying to achieve by giving us a runny nose, specifically during a cold.

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    $\begingroup$ Does it need to be evolved? Can it not just be a side effect of some inflammatory process? $\endgroup$
    – kmm
    Commented Sep 27, 2015 at 0:55
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    $\begingroup$ I don't know. That's the question. Tears seem to have a point. Coughing seems to have a point. Diarrhea seems to have a point. Fevers have a point. Vomiting seems to have a point. So, the question is does a runny noes have a point or as you suggest, is it just an unlucky side effect. $\endgroup$
    – gman
    Commented Sep 27, 2015 at 1:24
  • $\begingroup$ possible duplicate of If a trait would be advantageous to an organism, why hasn't it evolved? $\endgroup$
    – March Ho
    Commented Sep 27, 2015 at 2:33
  • 1
    $\begingroup$ This question is not asking why something hasn't evolved but, rather, why something has evolved. There's clearly potential for a scientific answer. In addition to AMR's answer, there's this idea: cmr.asm.org/content/24/1/210.full $\endgroup$
    – canadianer
    Commented Sep 27, 2015 at 3:04
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    $\begingroup$ @AMR It's disappointing being outsmarted by something that's not even alive. $\endgroup$
    – canadianer
    Commented Sep 27, 2015 at 4:20

1 Answer 1


Edited by request for clarity

The biological purpose of a runny nose is to clear pathogens and allergens trapped in the mucus out of the nasal cavity and sinuses as a result of rhinitis or sinusitis.

  • Mucus is considered to be part of the innate immune system and provides mechanical and chemical defenses against pathogens.
    • Mucus acts as a barrier protection that is difficult for pathogens to breach.
    • Mucus is capable of trapping pathogens in it and when that mucus is swept out of your upper respiratory track through your nose or down your esophagus, the pathogens are removed from those areas as well.
    • Defensive proteins called Mucins, Lysozymes, and Immunoglobulins are secreted into the mucus and provide additional protection, as they can lyse or opsonize pathogens.
  • Mucus keeps epithelial cells moist so that they do not dehydrate as air passes over them. This in turn keeps epithelial cells healthy so that they maintain a tight cell layer in order to protect the underlying tissue.

As part of the mechanical defenses against pathogens, epithelial cells are ciliated and they rhythmically sweep mucus from the sinuses and nasal cavity mainly into your esophagus and digestive system, but can also move mucus into your nose. Most of the time we aren't even aware this clearance of mucus is happening and it travels down our esophagus into our stomach's and intestines where it can be excreted from the body. This is the case when you do not have an infection and your mucus is not viscous.

When you have an infection the inflammatory response is triggered by cytokines from macrophages. Those cytokines induce cells in the area to do many things. One is to increase mucus production. This may cause epithelial cells in your nostrils and nasal cavity and sinuses to secrete more mucus as well, and this will be swept away through your nose, where you will notice it as a runny nose.

Usually your nose is runnier with rhinovirus infections or with allergens than it is when you have a bacterial infection where mucous tends to be thicker and more viscous. The ciliated epithelial cells have an easier time moving the thinner mucous of a viral infection or allergy than the thicker mucus of a bacterial infection, so you notice a runny nose more when you have a cold or hay fever.

So in short, the biological purpose of a runny nose is that it is part of the immune response to clear pathogens away and protect your uninfected epithelial cells from being infected. It also clears out much of the debris and inflammatory proteins that are found at the site of the infection so that they do not remain in the area to further inflame tissue that is trying to heal. When you have allergies your immune system reacts inappropriately to a benign substance, and if the allergen finds its way into your nasal cavity, it will generate rhinitis and cause your nose to run.

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    $\begingroup$ I agree with your last paragraph, but don't really follow the first. Why are you singling out the sinuses (do you think the main effect of a runny nose is to prevent sinusitis?) If so, that could be made more clear; if not, that should be made more clear as well. $\endgroup$ Commented Sep 27, 2015 at 2:17
  • $\begingroup$ @anongoodnurse I basically say sinuses because the mucus in you trachea and bronchi get swept up into your esophagus without going through your nose. For the most part the mucus that exits through the nose is from the sinuses and nasal cavity. $\endgroup$
    – AMR
    Commented Sep 27, 2015 at 3:25
  • $\begingroup$ @gman I will add that our interaction with and adaptations to pathogens are significant drivers to evolution of the species. $\endgroup$
    – AMR
    Commented Sep 27, 2015 at 16:05
  • $\begingroup$ Is this known through some sort of study, or conjecture? I ask because I've heard it said that some symptoms, such as sneezing, are not the body's way of fighting a virus, but instead an effect that the virus has evolved to induce in the body to aid in spreading itself more readily. A runny nose could be similar. I have no idea, but am curious whether anyone really knows the answer. $\endgroup$
    – Mark
    Commented Sep 28, 2015 at 1:05
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    $\begingroup$ Janeway's Immunobiology - 8th Edition. Chapter 2. $\endgroup$
    – AMR
    Commented Sep 28, 2015 at 1:21

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