I’ve struggled with various types of atopic dermatitis ― as well as dyshidrotic eczema, which causes tiny, agonisingly itchy bumps on my fingers ― for years now. 

While I’m able to resist scratching any affected patches while I’m out and about, I’ll admit I resent being told not to scratch the area in the blissfully private confines of my own home. “It’ll spread!”, my partner warns (I always try to argue dermatitis doesn’t work like that). 

Well, tough day for me ― according to a recent study from the University of Pittsburgh, I’m in the wrong. 

How does scratching affect rashes?

The scientists ran tests on mice to find out how scratching areas affected by allergic contact dermatitis, caused by contact with things like nickel and poison ivy, affected them. 

They discovered that the satisfying practice actually does increase inflammation in the area, delaying healing.

It led to inflammation in two ways: scratching caused a release of inflammatory immune cells called neutrophils, and it also led neurons to release a compound called substance P, which activated itch- and inflammation-causing P cells. 

Speaking to Science Direct, one of the papers’ authors, Dr Daniel Kaplan, professor of immunology and dermatology, said: “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, so the reason that scratching triggers more inflammation in the skin is because mast cells have been synergistically activated through two pathways.”

So why did we evolve to itch, then?

The urge to scratch an itch is pretty hard to resist. So why do we have it if it makes the condition worse?

Well, the scientists think that though the reaction may worsen the rash itself, it could also offer an immune response to certain bacteria. 

They found that scratching affected mice’s skin microbiome, making it more resistant to a common rash and itching-relation bacterial called staphylococcus aureus.

But alas ― though this might be helpful in the short term, Dr Kaplan told Science Direct that “the damage that scratching does to the skin probably outweighs this benefit when itching is chronic.”