Scratching an itch is satisfying, but is it good for your skin? (Pormezz/Shutterstock)
In a nutshell
- While scratching worsens allergic skin conditions like eczema by increasing inflammation, it also activates a defense mechanism against bacterial infections, particularly Staphylococcus aureus.
- Scratching activates mast cells through two pathways—substance P from pain-sensing neurons and allergens—which amplifies inflammation but also helps fight off harmful bacteria.
- The pleasure associated with scratching suggests it evolved as a protective behavior, balancing its harmful effects on inflammation with its ability to defend against skin infections.
PITTSBURGH — When you feel that irresistible urge to scratch an itch, you’re activating an evolutionary defense system. New research from the University of Pittsburgh reveals a fascinating paradox: scratching an itch makes allergic skin conditions worse, yet simultaneously helps fight off bacterial infections.
Published in the journal Science, this study sheds light on the complex relationship between scratching an itch and its effects on skin health. By studying how scratching interacts with the skin’s immune system, the researchers have revealed unexpected insights into this common yet misunderstood behavior.
“If scratching an itch is bad for us, why does it feel so good?” says lead study author Dr. Daniel Kaplan, Ph.D., from the University of Pittsburgh, in a statement. “Scratching is often pleasurable, which suggests that, in order to have evolved, this behavior must provide some kind of benefit. Our study helps resolve this paradox by providing evidence that scratching also provides defense against bacterial skin infections.”
The Vicious Itch-Scratch Cycle
If you’ve ever had eczema, you know how it goes: your skin itches, you scratch, it feels better for a moment but soon makes everything worse, leading to more itching. Doctors call this the “itch-scratch cycle,” and breaking it is incredibly hard. Unlike pain, which makes us pull away, itching triggers a desire to scratch, suggesting this behavior must have helped our ancestors survive.
Kaplan’s team found that scratching triggers pain-sensing nerve cells in the skin to release a chemical called substance P. This chemical activates mast cells, immune cells in the skin that help coordinate inflammatory responses.
“In contact dermatitis, mast cells are directly activated by allergens, which drives minor inflammation and itchiness,” explains Kaplan. “In response to scratching, the release of substance P activates mast cells through a second pathway, so the reason that scratching triggers more inflammation in the skin is because mast cells have been synergistically activated through two pathways.”
When mast cells get activated through both pathways at once, they create a stronger inflammatory response that brings neutrophils, white blood cells that fight infections but also cause swelling and redness, to the area.
The Research Behind the Itch
To study this process, Kaplan’s team used allergens to create eczema-like symptoms on the ears of two groups of mice: normal ones and special mice engineered to not feel itchy because they lack itch-sensing nerves.
Nate Langer, UPMC)
When normal mice scratched freely, their ears became swollen and full of inflammatory cells. But inflammation was much milder in mice that couldn’t scratch because they wore tiny “cones of shame” (like dogs wear after surgery), and in the special mice that couldn’t feel the itch. This showed clearly that scratching makes skin inflammation worse.
The researchers found that scratching starts a chain reaction that increases inflammation far beyond what the initial allergic reaction would cause by itself.
When Scratching Helps: The Bacterial Connection
The research took an unexpected turn when the team examined bacterial infections. They discovered that scratching reduced the amount of Staphylococcus aureus (staph), a common skin bacteria, on the skin.
“The finding that scratching improves defense against Staphylococcus aureus suggests that it could be beneficial in some contexts,” says Kaplan. “But the damage that scratching does to the skin probably outweighs this benefit when itching is chronic.”
Scratching changes the mix of bacteria on the skin. In mice exposed to staph bacteria, scratching significantly reduced bacterial numbers. This bacteria-fighting benefit was not found in mice that couldn’t feel the itch or were prevented from scratching.
In experiments led by coauthor Marlies Meisel, Ph.D., assistant professor of immunology at Pitt, the team demonstrated how scratching affected the skin’s bacterial community, reducing certain types of bacteria while helping clear harmful ones.
What This Means for Treatment
Current eczema treatments focus on reducing inflammation and stopping itching. While this helps symptoms, completely blocking scratching or inflammation might accidentally interfere with the body’s ability to fight certain infections.
By understanding exactly how these nerve and immune pathways work together, scientists could develop smarter treatments that reduce harmful inflammation while preserving beneficial defense functions. The researchers are now working on new therapies for conditions like eczema, rosacea, and hives that target specific receptors on mast cells.
The next time you feel itchy, remember that scratching an itchy rash does make it worse, but this behavior is also part of an ancient defense system that has helped protect human skin from harmful bacteria throughout our evolutionary history.
Paper Summary
Methodology
The researchers created genetically modified mice (called Mrgpra3DTR mice) that allowed them to selectively eliminate specific itch-sensing neurons. They also created mice with receptors that could temporarily disable pain-sensing neurons. Using these mice, the team studied how scratching affected skin inflammation in models of allergic skin reactions and bacterial infections. They measured ear thickness, immune cell infiltration, mast cell activation, and bacterial levels on the skin.
Results
The team found that scratching activated pain-sensing neurons, which released substance P – a chemical that activates mast cells through a receptor called MrgprB2. When mast cells were simultaneously activated by both substance P and allergens, they produced a stronger inflammatory response, including increased TNF release and neutrophil recruitment. However, this same inflammatory response helped clear Staphylococcus aureus infections. Mice that couldn’t scratch showed reduced inflammation but also reduced ability to fight bacterial infections.
Limitations
This study was conducted entirely in mice, whose skin differs from human skin in important ways. The research focused on acute inflammation models rather than chronic conditions like atopic dermatitis. The researchers used genetic techniques and physical barriers to prevent scratching, which may not perfectly mirror the effects of anti-itch medications in humans. While scratching provided protection against S. aureus in their model, in chronic skin conditions, S. aureus overgrowth is common despite scratching, suggesting other factors are involved.
Discussion and Takeaways
This research provides a mechanistic explanation for the “itch-scratch cycle” in allergic skin diseases and reveals why scratching evolved despite its downsides. The findings suggest that completely blocking itch or inflammation might compromise the body’s ability to fight certain infections. Future treatments might selectively target harmful aspects of inflammation while preserving beneficial host defense functions. The research highlights the complex relationship between the nervous system, immune system, and skin microbiome.
Funding and Disclosures
This research was supported by grants from the National Institutes of Health (T32NS73548, T32CA0820840, R01DK130897, U24EY035102, K99NS126569, R01AR071720 and R01AR077341) and the German Research Foundation. Several authors disclosed potential conflicts of interest, including patents filed by the University of Pittsburgh describing compounds to suppress mast cell function, and consulting relationships with pharmaceutical companies.
Publication Information
The paper, “Scratching promotes allergic inflammation and host defense via neurogenic mast cell activation,” was published in the January 31, 2025, issue of Science (Volume 387, Issue 6733). The research was conducted by a team led by Dr. Daniel H. Kaplan at the University of Pittsburgh, with first author Andrew W. Liu and numerous contributing researchers from the departments of Dermatology, Immunology, and Anesthesiology.
