Nail’d it!
Scratching an itchy rash can feel sooo good — that’s because the rubbing creates a mild pain sensation that distracts the brain from the itch. The pain triggers the brain to release serotonin, the “feel-good hormone.”
A new study reveals scratching a rash has other upsides — along with the downsides your mom warned you about.
“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,” said senior study author Dr. Daniel Kaplan, a professor of dermatology and immunology at the University of Pittsburgh.
For the research, Kaplan’s team was able to mimic allergic contact dermatitis on the ears of mice.
The condition occurs when skin comes into contact with a substance that causes an allergic reaction, such as poison ivy or metals like nickel in jewelry.
Some of the mice were allowed to scratch their rash. Their ears became swollen and filled with neutrophils, a type of white blood cell that helps the body fight infection.
The mice that couldn’t scratch had much less inflammation and swelling, confirming that scratching further aggravates the skin and slows healing.
The researchers also observed that scratching causes neurons that sense pain to release a chemical called substance P, which activates mast cells.
Mast cells are immune cells that release chemicals when they encounter allergens. The chemicals include histamine, which causes swelling and redness at the site of the allergic reaction.
“In contact dermatitis, mast cells are directly activated by allergens, which drives minor inflammation and itchiness,” Kaplan explained. “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.”
On the plus side, the researchers noted that mast cells protect against bacteria and other pathogens.
Further experimentation showed that scratching reduced the amount of Staphylococcus aureus on the skin. The bacteria, also known as staph, is the most common cause of skin infections and can lead to food poisoning, pneumonia and bone infections.
Kaplan’s results were published Thursday in the journal Science.
“The finding that scratching improves defense against Staphylococcus aureus suggests that it could be beneficial in some contexts,” Kaplan said. “But the damage that scratching does to the skin probably outweighs this benefit when itching is chronic.”
Kaplan is building upon this knowledge to investigate new therapies for inflammatory skin conditions like dermatitis, rosacea and hives.
