Immune Science Behind the Satisfaction of Scratching

Scratching an itch often brings immediate relief, but a groundbreaking study published in Science has revealed there’s more to it than simple pleasure.

Research on mice shows that scratching not only heightens inflammation but also offers immune benefits, such as reducing harmful bacteria on the skin. These findings provide a molecular explanation for the urge to scratch, which may have evolved as a defense mechanism against infections.

Scratching is a near-universal behavior among animals. The traditional explanation has been that it helps remove parasites or irritants. However, scientists have long suspected that additional factors are at play, especially since itching often starts after parasites like mosquitoes have already left.

Dan Kaplan, a skin immunologist at the University of Pittsburgh, led a study that investigated the immune effects of scratching. His team applied an allergen to the ears of mice to trigger a type of inflammation similar to allergic contact dermatitis in humans. Mice that were allowed to scratch developed more swelling and showed an increase in immune cells called neutrophils compared to those that wore protective collars to prevent scratching. This suggested that scratching itself plays a role in promoting inflammation.

The study found that scratching causes pain-sensing neurons to release a messenger molecule called substance P. This neuropeptide activates mast cells, which are key players in allergic reactions. The activated mast cells recruit neutrophils, ramping up inflammation.

Interestingly, the researchers discovered that mast cells could be activated both directly by allergens and indirectly through the act of scratching. This dual pathway was surprising, said Kaplan.

The study also revealed a beneficial side to scratching. When mice were exposed to an allergen, those that scratched were less likely to have harmful bacteria, such as Staphylococcus aureus, on their skin compared to those prevented from scratching. This suggests that scratching may help defend against bacterial infections.

Kaplan emphasized that these findings apply to acute itching rather than chronic cases. Chronic scratching can damage the skin, making it more vulnerable to infections like S. aureus.

The research offers hope for people suffering from chronic itching, which can be caused by conditions such as eczema and diabetes. Scientists found that the body has separate nerve pathways for sending itch signals and for increasing inflammation in response to scratching. If these pathways can be decoupled, it may be possible to develop treatments that block one without affecting the other.

“There’s this really vicious itch-scratch cycle that can’t be broken,” said immunologist Aaron Ver Heul, who was not involved in the study. “Identifying a circuit like this hopefully will lead to therapeutics that will be better at breaking the cycle.”

Nature and Science contributed to this report.