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Universal human behavior -

Scratching an itch is a nearly universal human behavior, providing momentary relief but often leading to increased inflammation. While scratching is pleasurable, its consequences can be detrimental, particularly for individuals with chronic skin conditions.

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A new study published by Science -

However, recent findings reveal that scratching also has positive effects. According to the scientists involved in this study, published by Science, scratching activates an immune response that helps protect the skin against harmful infections.

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A double edged sword -

Daniel Kaplan, MD, PhD, senior author of the study and professor of dermatology and immunology at the University of Pittsburgh, highlights this paradox: “Scratching is often pleasurable, which suggests that, in order to have evolved, this behavior must provide some kind of benefit.”

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Pleasure and pain -

This paradox is particularly evident in chronic skin conditions where patients experience an overwhelming urge to scratch despite knowing it worsens their symptoms. The pleasurable sensation associated with scratching is due to the activation of brain regions involved in reward and addiction, including the release of dopamine.

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A reinforcing cycle -

This creates a reinforcing cycle where individuals continue to scratch even when it leads to further damage. The study aimed to resolve this contradiction by exploring the physiological mechanisms behind scratching and its effects on inflammation and immune defense.

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Breaking the cycle -

Understanding the neurological basis of this paradox could lead to better management strategies for conditions like eczema and psoriasis, where breaking the itch-scratch cycle is crucial for symptom control.

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Allergic contact dermatitis on mice -

To understand the relationship between scratching, inflammation, and immunity, researchers painted a synthetic allergen on the ears of mice. This induced a form of skin inflammation called allergic contact dermatitis, characterized by an allergic reaction to irritants such as poison ivy or nickel.

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Observations -

The study included both normal mice and genetically modified mice that lacked nonpeptidergic 2 (NP2) itch-sensing neurons, which are responsible for transmitting itch signals. Researchers observed that normal mice who were allowed to scratch and subjected to allergens exhibited severe inflammation, with increased swelling and infiltration of immune cells called neutrophils.

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Observations -

However, in mice that either lacked itch-sensing neurons or were prevented from scratching by wearing protective collars, inflammation remained significantly milder. This confirmed that scratching itself amplifies the inflammatory response.

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The role of substance P and mast cells -

Further analysis revealed that scratching activates pain-sensing neurons, leading to the release of a neuropeptide called substance P (SP). This compound, in turn, stimulates mast cells, a type of immune cell involved in allergic reactions and inflammation.

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The role of substance P and mast cells -

Mast cells play a critical role in recruiting neutrophils, which contribute to swelling and irritation. Researchers found that scratching-induced inflammation was dependent on SP, pain-sensing nociceptors, and the mast cell receptor MrgprB2.

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Dual pathway -

“In contact dermatitis, mast cells are directly activated by allergens, which drives minor inflammation and itchiness. In response to scratching, the release of substance P activates mast cells through a second pathway,” Kaplan explained.

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The itch-scratch feedback loop -

Therefore, the reason scratching triggers more inflammation is that mast cells are synergistically activated through two mechanisms. This dual activation not only amplifies the inflammatory response but also prolongs the itch-scratch cycle, creating a feedback loop that exacerbates skin irritation and delays healing.

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Influencing inflammation and immunity -

Moreover, the study revealed that the activation of mast cells by scratching influences not only inflammation but also the broader immune landscape of the skin. When mast cells are stimulated by substance P, they release histamines and other inflammatory mediators such as cytokines and prostaglandin D2.

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A dual purpose -

These substances amplify the recruitment of immune cells like neutrophils and eosinophils, which contribute to tissue swelling and redness. However, this heightened immune activity could serve a dual purpose: while it worsens inflammation in allergic reactions, it may also create an inhospitable environment for harmful microbes attempting to colonize the skin.

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Scratching as a microbial defense mechanism -

Despite its role in inflammation, scratching may offer an unexpected advantage: protection against bacterial infections. Scratching lessened the amount of potentially harmful bacteria on the mice's skin.

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Potential to influence skin microbiome -

The researchers hypothesized that the activation of mast cells might influence the skin microbiome, particularly in relation to Staphylococcus aureus, a bacterium frequently implicated in skin infections and atopic dermatitis.

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Scratchers had lower levels of Staphylococcus aureus -

In experiments led by co-author Marlies Meisel, PhD, assistant professor of immunology, the team found that mice allowed to scratch had lower levels of Staphylococcus aureus on their skin compared to those prevented from scratching.

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Possible reduction of bacterial colonization -

This suggests that scratching, while damaging to the skin barrier, may help reduce bacterial colonization, potentially preventing infections. An important caveat, Kaplan stressed, is that the work focused on acute rather than chronic itch. Chronic scratching can lead to skin damage, giving Staphylococcus aureus more of a foothold.

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A blissful sensation -

Unlike pain, which triggers aversive behavior, scratching an itch is “often a pleasurable sensation” and does not trigger avoidance. This suggests that scratching may have evolved not only as a response to discomfort but also as a mechanism to maintain skin integrity and reduce microbial threats.

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More to consider -

Beyond Staphylococcus aureus, the impact of scratching on the skin microbiome may extend to other bacterial and fungal species. Researchers speculate that scratching-induced immune activation could disrupt biofilm formation, a protective mechanism used by bacteria to resist immune responses and antibiotic treatments.

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Future studies -

This raises intriguing questions about whether controlled scratching, in moderation, might help regulate microbial diversity on the skin, reducing the likelihood of chronic infections. Future studies may explore whether specific scratching behaviors such as frequency, intensity, or duration, play a role in shaping the skin’s microbial environment.

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Implications for dermatology and future treatments -

This discovery has significant implications for treating chronic inflammatory skin diseases such as eczema, rosacea, and urticaria. Although excessive scratching worsens inflammation, its ability to enhance bacterial defense raises questions about how therapeutic interventions can balance these effects.

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Pharmacological strategies -

The researchers are now exploring pharmacological strategies to selectively suppress the inflammatory consequences of scratching, while preserving its immune-boosting benefits. By targeting mast cell receptors or modulating substance P signaling, future treatments may mitigate the harmful aspects of scratching while harnessing its protective properties.

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Patient education -

These findings suggest that dermatologists may need to reconsider how they approach patient education about scratching. While it is widely advised to avoid scratching to prevent worsening inflammation, a more nuanced approach may be needed, particularly for individuals prone to recurrent bacterial skin infections.

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Targeted therapies -

This also highlights the potential for targeted therapies that inhibit the inflammatory aspects of scratching without compromising its protective effects. Treatments that regulate mast cell activity without completely suppressing immune responses could be a promising avenue for future research.

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A neuroimmune response with dual consequences -

Scratching an itch is more than a reflexive response; it is a complex neuroimmune interaction with both positive and negative consequences. While excessive scratching exacerbates inflammatory skin diseases, it also serves as a primitive defense mechanism against bacterial threats.

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Innovative dermatology therapies -

Understanding these dual effects opens new avenues for dermatological research, potentially leading to innovative therapies that address chronic itch and skin inflammation while preserving the skin’s natural ability to defend itself against pathogens.

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Relief and resilience -

This study ultimately reframes scratching not as a mere nuisance but as a behavior with evolutionary significance. One that has persisted because it provides both relief and resilience against microbial threats.

Sources: (Science) (Nature) Gen News) (New Scientist)

See also: Sensitive skin: causes, care, and when to seek help

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