First, we'll answer the easy portion of your question. A runny nose – formally known as rhinorrhea – occurs when nasal passages produce excessive amounts of mucus. This can happen because it's cold (the cold air dries out the nasal cavity, and your body overcompensates) because you're crying (excess liquid from the tear ducts drains out into the nasal cavity) or because your immune system is responding to an invader (more on that in a second).
As extra mucus builds up in your nose, it makes it difficult to breath and can even leak its way into the sinuses or ear canal and cause irritation and infections there. It can also drip into the back of the throat, causing the soft tissue to become sore. In other words, your stuffy nose is often the root cause of all your other cold and allergy symptoms.
So why on Earth do our bodies betray us this way?
"I know it’s very uncomfortable for many people, but a lot of the symptoms [of colds and allergies] are in a certain way protective," Dr. Kate Welch, a physician specializing in allergy and immunology at Weill Cornell Medicine and NewYork-Presbyterian, told The Post. "They help us to expel things, whether we're talking about a runny nose or a physical expulsion like sneezing or vomiting."
That leads us to the next part of your question: Why allergies exist. Let's be real; allergies suck. An allergy, by its definition, is the body's reaction to a substance that most people can encounter without getting ill. Hundreds of millions of people in the United States will happily and safely chow down on peanut butter, for example, but some 3 million individuals suffer everything from itchy hives to deadly throat swelling if they consume the wrong legume.
The scientific jury is still out on why, exactly, some people develop such severe allergies to seemingly harmless substances. In some cases, it's possible that – for whatever reason – the item isn't actually harmless to that individual. While tell-tale allergic reaction symptoms like swelling, itching, sneezing and vomiting are definitely caused by the immune system (as opposed to being caused by the allergen itself) it's hard to determine what kind of ill effects a peanut or piece of shellfish might have on your body when you avoid it because of a violent allergy.
But in most cases, allergies are likely just the result of an over-eager immune system. Antibodies called Immunoglobulin E (IgE) pop up with the sole purpose of triggering the release of histamine – a compound that inflames your tissues and causes leaky noses, among other things.
"The basic alarm mechanism is very helpful, but it’s kind of a fine line between which ones help and which ones don’t," Welch explained.
Many experts believe that this overactive IgE is a holdover from immune responses that helped save our ancestors' lives. The molecules involved are quite similar to the ones that protect humans and other animals from venom and parasites. Perhaps when our species was a little more wild, we were exposed to more genuinely harmful pollen-like substances, making allergic reactions a real lifesaver.
"Back in the day when we were hunter gatherers, certain pollens or venoms may actually have been very dangerous to us," Welch said. "So we had this mechanism to help us expel them." Even the annoyance of allergies might have had an evolutionary benefit at one time. "Particularly uncomfortable responses might drive us out of the area," Welch said. "It's like the immune system is saying, 'hey, get away from these bees!'"
"Evolution doesn’t really care about how you feel about the symptoms," she added.
Over time, these responses – which worked so well that they kept kicking around the gene pool – got exaggerated. And then, at least according to some hypotheses, our relatively quick cultural evolution left IgE desperate for an outlet.
It's possible that IgE found its creative outlet in the form of pollens – and foods – that resemble old toxins on the molecular level. Lots of people with pollen allergies get itchy mouths when they eat certain foods, which seems to Welch like an indication that biological wires are getting crossed.
Even with all that miss- and over-firing, the immune systems histamine responses still serve a purpose every once in awhile. After all, you want to have a body that rejects genuinely harmful substances. So scientists would never try to get rid of allergic reactions across the board.
"It’s nice to have an alarm system, but if we can dampen it down when it’s not actually dangerous that would be great," Welch said. "But the underlying point is that all of these uncomfortable mechanisms do serve a purpose. It serves some role, it just doesn’t make us very comfortable."