Spit heals wounds: White blood cells in saliva mucus.

How Spit Heals Wounds

How Spit Heals Wounds

The state of the universe.
Nov. 17 2015 9:30 AM

How Spit Heals Wounds

Next time you wake up in a puddle of drool, don’t be repulsed, be thankful.

spit is antiseptic.
A man making use of the healing powers of saliva.

Photo illustration by Juliana Jiménez. Photo by George Doyle/Thinkstock.

Saliva is a glorious thing. Not only is it good for licking lollipops, sealing envelopes, moistening bites of food, and making spitballs—but it actually helps wounds heal faster. Now researchers at Lund University in Sweden have elucidated the mechanism that gives your saliva its extraordinary powers of healing.

We’ve known for years that saliva does more than physical cleansing. When you suck on a cut finger, white blood cells in your spit release an arsenal of weapons to defeat bacterial invaders. First, white blood cells can simply engulf the bacteria and die as martyrs. Or they can shoot bacteria with deadly substances that cause the bacteria to disintegrate. But white blood cells can also do something even cooler: create sticky, viscous “nets” and throw them out like lassos to capture and wrestle bacteria to their deaths.

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Now we know that the nets created by your spit are better suited for battling bacteria than those made by white blood cells in other parts of the body, according to a team led by Ole Sorensen whose findings were published this month in the journal Blood.

Initially, Sorensen had been looking at how wounds heal by investigating bits of cultured skin tissue. Then he had a crazy idea. He knew that licking wounds helped them heal, and that wounds in the mouth heal faster than wounds anywhere else in the body—despite the fact that we have a teeming population of bacteria in the mouth. But no one knew the exact mechanism. So he took the cultures of wounded tissue, “and then we sort of spat on them,” he says.

Besides being kind of gross, that impulse was also a little weird scientifically. See, the enzymes in saliva are known to digest the kinds of nets that white blood cells cast, leaving few traces of their presence. As a result, you wouldn’t typically be able to detect whether white blood cells had released nets just by going in afterward and checking. But when Sorensen examined the spit-covered wounded skin cells, he found thousands and thousands of nets, perfectly intact. How?

The first step in finding out was to collect lots of drool. The researchers had people donate tubes of “morning saliva” collected just after awakening and before brushing their teeth. The “morning” part is important: During the day, your mouth secretes up to half a gallon of spit, most of which you reflexively swallow. But while you slumber, swallowing ceases. That accumulation of spit in your mouth creates, besides the puddle of drool that graces your pillow in the morning, a wealth of tiny nets.

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The key, researchers found, was in the mucus—that mixture of white blood cells, water, salt, dead cells, and the carbohydrate-coated protein mucin that creates the stringy, clumpy stuff in your saliva. When the researchers distilled out the protein-containing mucus from the rest of the saliva and added it to white blood cells, the white blood cells immediately released their nets.

But these were no ordinary nets. In this case, the white blood cells, called neutrophils, went through different steps to create the nets. Moreover, the nets themselves had a distinct protein composition compared with those created by other white blood cells, making them better protected against enzymes that would otherwise destroy them. In Sorensen’s words: “They were wearing bulletproof vests.” He surmised that these nets could complement wound healing on the skin by killing off the remaining bacteria that white blood cells in the wound don’t engulf and destroy.

Sorensen also suspected that this was the saliva’s best form of defense, rather than just one of many. To test how it might work or fail in living people, he collected the saliva of patients with recurring oral ulcers. In every instance, it lacked the component of mucus that produced the nets. The people’s cycles of oral sores coincided with cycles of having the net-producing protein. When their saliva returned to its net-producing state, the wounds began to heal again.

“The discovery that neutrophils release nets upon exposure to salivary mucins is novel and exciting,” says Jeremy Barr, a researcher at San Diego State University who discovered a novel and separate immune system in human mucus and was not involved in the research. “This discovery suggests that the oral cavity is better adapted to protect against infection than we previously thought, and may provide us new ways of combatting oral diseases.”

For instance, Sorensen hopes to help develop a replica of the key mucus components—kind of like a spit transfusion. Such a substance might rearm the defenseless spit, giving people the ammunition to lasso bacteria into submission. And, with luck, it’s goodbye ulcers!

So the next time you wake up to a sticky trickle of drool on your pillow, don’t be repulsed; be thankful. The more we learn about our mucus, the more we have to appreciate about our salivary savior.

Rachel E. Gross is the science web editor at Smithsonian.