The itch from bug bites, rashes, and other skin conditions can sometimes be so overpowering that it feels impossible to avoid scratching them. But new research explains why you might want to hold off as long as you can.
Scratching is a bit of a conundrum biologically. It can feel extremely good, which would make you think that we evolved to engage in the behavior for some kind of beneficial reason. Yet, as most of us know, scratching a mosquito bite or a skin rash always makes it worse. Such behavior can break the skin, potentially opening us up to infection.
Researchers at the University of Pittsburgh set out to tackle this paradox and determine if scratching is indeed as bad as it seems but also to find out if it serves any positive purpose.
So they exposed some unlucky mice to itch-inducing allergens to create eczema-like symptoms on their ears. Some of these mice were allowed to scratch all they wanted, while others were modified to lack itch-sensing neurons in the area, and a third group wore scratch-preventing Elizabethan collars, similar to what are often referred to as "cones of shame" when worn by dogs.
Observation not only revealed the expected result that the ears of the mice which could scratch themselves were worse than the other two groups, but also uncovered why.
The researchers say that in response to scratching, pain-sensing neurons released an amino acid chain referred to as "substance P." This, then, caused the activation of mast cells, immune cells found throughout the body. The activation of mast cells can frequently lead to inflammation and the itch response thanks to their recruitment of neutrophils, a type of white blood cell also involved with the body's immune response.
In short, this chemical and cellular cascade is what causes an itchy spot on the skin to get angry and inflamed when scratched.
“In contact dermatitis, mast cells are directly activated by allergens, which drives minor inflammation and itchiness,” said senior study author Daniel 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.”
Any benefit?
That being said, because mast cells are also involved in innate immunity, the researchers wondered if scratching's role in activating them conveys some kind of infection-fighting benefit.
Sure enough, they found that scratching did in fact reduce the amount of Staphylococcus aureus on the skin. S. aureus is the most common bacteria involved in skin infections and is most famously responsible for staph infections.
Still, in the research team's opinion, the reduction in this bacteria from scratching doesn't outweigh the way in which the behavior worsens the area around an itch.
“The finding that scratching improves defense against Staphylococcus aureus suggests that it could be beneficial in some contexts,” said Kaplan. “But the damage that scratching does to the skin probably outweighs this benefit when itching is chronic.”
Or, put another way, as your mom would say: "Stop scratching!"
The findings have led the research team to continue down the path of mast-cell research, specifically the way in which new therapies might be developed for inflammatory skin conditions like rosacea by targeting receptors on the cells.
The current research has been published in the journal Science.
Source: University of Pittsburgh
