The urge to scratch an itch is hard to resist. People with eczema, also known as allergic contact dermatitis, know this all too well.

Exposure to allergens or irritants like poison ivy, cleaning products, disinfectants, lotions or certain types of jewelry (usually containing nickel) can cause their skin to break out. Giving in to the urge to scratch this itchy, red rash triggers more inflammation and slows down the healing process.

Mast cells act as agents in inflammatory skin conditions, but they also protect the skin against bacteria and other pathogens. It is this protective effect that makes the urge to scratch so irresistible.

To investigate the itch-scratching cycle in more detail, researchers from the University of Pittsburgh applied allergens to the ears of mice to induce dermatitis, a form of skin inflammation. In a series of experiments led by study coauthor Marlies Meisel, an assistant professor of immunology at Pitt, some of the mice were allowed to scratch their ears freely; others were prevented from scratching their ears using an Elizabethan collar.

Mice able to scratch experienced more inflammation and also a stronger immune response. At the same time, the mice allowed to scratch were less likely to harbor Staphylococcus aureus, a form of bacteria known to cause skin infections, than mice that couldn't.

The researchers were particularly interested in the role mast cells, a type of immune cell, play in this cycle. Scratching activates mast cells, and they found that when these immune cells become activated, they release itch-inducing factors called pruritogens that are sensed by neurons expressing a receptor called MgrprA3.

Scratching the resulting itch causes neurons expressing a receptor called Trvp1 to release the neurotransmitter substance P. This neurotransmitter causes mast cells to release tumor necrosis factor, which recruits immune cells called neutrophils to cause further inflammation and swelling.

Scratching an itch triggers more inflammation because mast cells have been activated through two different pathways, the MgrprA3 and Tryp1 pathways, senior author Daniel Kaplan explained in a statement. Mast cells are agents in inflammatory skin conditions. But the research revealed they also protect the skin against bacteria and other pathogens. This protective effect makes the urge to scratch so satisfying and irresistible. Their findings were published recently in Science.

The best way to prevent itching in the first place is to take good care of your skin. Use emollients to keep the skin barrier from drying out and cracking, which allows allergens to get through.

While scratching appears to have some benefits, “...the damage scratching does to the skin probably outweighs this benefit when the itching is chronic,” said Kaplan, a professor of dermatology and immunology at Pitt.

This new insight into the itch-scratch cycle could provide targets for new medications to treat inflammatory skin conditions. These medications could suppress inflammation by targeting receptors on mast cells.

“Drugs are in the pipeline that can target mast cells better than we have ever been able to target them before,” Aaron Ver Heul, the author of an editorial related to the study, told TheDoctor.

The best way to prevent itching in the first place is to take good care of your skin, he added. Use emollients to keep the skin barrier from drying out and cracking, which allows allergens to get through.

Avoid products to which the skin can be sensitive, such as those containing dyes, fragrances and preservatives. “I always recommend people use hypoallergenic skin products,” Ver Heul, an assistant professor of allergy and immunology at Washington University in St. Louis, said.

People with sensitive skin should apply a quarter-size test patch of a new skin care product to the inside of their arm daily for four weeks. This will avoid triggering massive inflammation and may help the body adjust to the new substance.