Mom knew what she was talking about after all: Scratching really does make the itch worse. Good as it feels, the burst of ecstasy you get from clawing your irritated skin only prolongs a vicious itch-scratch cycle, putting true relief farther out of reach. Our natural instinct betrays us.
But why? Though itch has bedeviled our species (not to mention many other animals) for thousands of years, scientists have just begun to comprehend the physiological mechanisms behind it. Over the past couple of decades, research has shown how scratching taps into our brain’s reward and immune systems, producing a strange mix of pleasure and discomfort that makes it all but impossible to resist.
“You scratch to feel better,” says Brian Kim, a neuroimmunologist at Mount Sinai’s Icahn School of Medicine, “but in doing so, you actually activate immune pathways that are counterproductive.” For most of us, this feedback loop is merely annoying, but it can become debilitating for the 1 in 5 people who suffer from chronic itch at some point in their lives.
What Is Itching, Anyway?
In 1660, a German physician named Samuel Hafenreffer gave the definition of itch (pruritus, in medical jargon) that’s still in use today: an unpleasant sensation that provokes the desire to scratch. It comes about through everything from insect bites and poisonous plants to allergic reactions and skin conditions like eczema.
Until fairly recently, itch was considered a form of mild pain arising from the weak activation of nociceptors or pain receptors. This “intensity theory” speculated that itch stood at one end of the pain spectrum, and would transition all the way to full-blown agony as a stimulus grew stronger.
But then, in 2007, an itch researcher at Washington University named Zhou-Feng Chen discovered a cluster of nerve cells in the spinal cord that are designated to transmit signals for itch and itch alone. These receptors are completely distinct from nociceptors, meaning that, although itch is deeply intertwined with pain, it’s a sensory experience unto itself.
“That allowed us to say [itch] is a real thing,” says Kim, who worked with Chen for years before starting his own lab. “This isn't just mild pain; this isn’t something nebulous.” Nevertheless, the link between the two sensations would be crucial to explaining why an itch always seems to come back with a vengeance.
Read More: There's So Much More To Explain About How Bodies Sense Pain
Why Does Scratching an Itch Feel Good
A few years later, in 2014, Chen and his colleagues realized that serotonin (the mood-regulating “happiness hormone”) plays a big role in perpetuating the itch-scratch cycle. They found that when mice scratch an itch, their brains release a surge of serotonin, resulting in a moment of pure bliss — but then, tragically, the hormone activates the same receptors that transmitted the itch signal in the first place, restarting the process.
These results jibed with a PLOS One brain-imaging study from the previous year, which showed that scratching activates the brain regions involved in the reward circuit, a neural network that floods the body with euphoria-inducing chemicals in response to pleasurable activities (like sex) and substances (like drugs). It’s the same system that underlies addiction, which isn’t surprising if you’ve ever lost an hour digging into full-body mosquito bites with rapturous abandon.
So much for the positive reinforcement aspect of scratching. But, getting back to the connection between itch and pain, Kim notes there’s another side to this coin: negative reinforcement, the longing not to feel pleasure but to be rid of discomfort. “That’s what’s so complex about it,” he says. “There are different levels at which you need to scratch.”
As it turns out, what suppresses itch is not scratching itself but rather the subtle soreness that follows scratching. Each time you drag your nails across your skin, you damage the outer layer of the epidermis ever so slightly, and a new sensation (mild pain) distracts you from the old one. It may seem odd to feel grateful for this pain — masochists excluded — but apparently, we consider itch the greater evil.
Unfortunately, that skin damage also prompts an immune response. As the brain receives the incoming pain signals, it instructs the body to release histamine (or other itch-inducing chemicals, as the case may be) at the offending site. Inflammation ensues, and what probably started as a minor problem is blown out of proportion. Before you know it, you’re off to the scratch races.
Read More: Why Does Pain Hurt?
What’s the Point of Itching?
You might be thinking that this frustrating cycle doesn’t make much sense. Pain (itch’s next of kin) exists to steer us away from danger; if something hurts, we instinctively avoid it and thus prevent further bodily harm. The evolutionary rationale for itch, on the other hand, isn’t clear at first glance.
Say you’re an ancient (or modern, for that matter) human living in a parasite-infested region. Many of those parasites are transmitted via insects, so natural selection will tend to favor defense mechanisms that ward off the pests. By swatting mosquitoes — a reflex against the itch we feel when they land on us — we dodge whatever diseases they may carry. Essentially, Kim says, “You are vaccinating yourself against malaria.”
Itch, in his telling, is sort of like taxes: A dreadful inconvenience that doesn’t necessarily benefit you in obvious ways. You may not always understand it, you may at times deeply resent it, but just like a government-funded highway, it does benefit you nonetheless.
From this perspective, not only itch but also the itch-scratch cycle may improve your chances of survival. If you’ve been bitten, it means there are disease vectors nearby, and constant fidgeting is more likely to keep them at bay. No one said life was fair. (This may also help explain the phenomenon of contagious itch since anyone who mirrors the scratching behavior will get the same benefit, regardless of whether they themselves have been attacked.)
Read More: Why Do Mosquitoes Love Biting Some People More Than Others?
Living with Chronic Itch
For most of us, itches are temporary. They come and go, leaving us in peace after a moment or, at most, a few days. But millions of people suffer from chronic itch, and even the most satisfying scratch is a small recompense for the long-term suffering their conditions bring. As the Buddhist philosopher Nagarjuna put it, “There is pleasure when an itch is scratched, but to be without an itch is more pleasurable still.”
In the worst cases, pleasure disappears altogether. Itch — and the ever-escalating itch-scratch cycle — can become almost intolerable, ruining a person’s sleep, relationships, and overall happiness. Even so, it’s historically understudied, and there’s been a corresponding lack of investment in medical treatments. But as scientists become better acquainted with the physiological pathways behind itch, they may be able to identify drug targets to neutralize the troublesome sensation.
There’s still much to learn. Ethan Lerner, who runs an itch research lab at Massachusetts General Hospital, wrote in a 2021 review that “we are at the proverbial end of the beginning with respect to having a deep understanding of itch.” Still, he adds, “There is now the potential to markedly alleviate chronic itch, a condition that does not end life but often ruins it.”
Read More: Scratching Away: The Complexities of Chronic Itch
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Neuroimmunologist at Mount Sinai’s Icahn School of Medicine. Brian Kim
Current Problems in Dermatology. Epidemiology of Itch
Nature. A gastrin-releasing peptide receptor mediates the itch sensation in the spinal cord
PLOS One. Brain’s Reward Circuits Mediate Itch Relief. A Functional MRI Study of Active Scratching
Frontiers in Human Neuroscience. Contagious itch: what we know and what we would like to know
American Physiological Society Journal. Physiology and Pathophysiology of Itch
Cody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor's degree in journalism and media production from Washington State University.
