Study: What Causes Skin to Itch?

person Itch

It concerns one of those actions that are hardly avoidable: when it itches, it is very difficult to resist the urge and refrain from scratching. Even the mere thought of an uncomfortable wool sweater, a mosquito bite, or even head lice can trigger the sensation of itching. Additionally, observing someone else scratching automatically evokes the impulse to scratch ourselves.

Originally, this reflex served as a warning signal: the body indicates that something harmful or toxic could be on our skin. Scratching, at least, provides a chance to remove it as quickly as possible. Scratching also alleviates the itching, albeit temporarily, and activates the reward system, making us feel better.

However, the molecular mechanisms behind itching remain largely unclear, as do the connections with skin conditions such as eczema and atopic dermatitis, which often involve distressing itching. Researchers from Harvard Medical School have delved into the phenomenon, demonstrating for the first time that a widespread skin bacterium, Staphylococcus aureus (S. aureus), can cause itching by directly affecting nerve cells.

Chain Reaction

Previously, it was believed that the itching associated with eczema and atopic dermatitis was caused by the accompanying inflammation of the skin. The experimental study, based on investigations on mice and human cells and published in the journal Cell, now suggests that S. aureus is solely responsible for this itching. In skin diseases, the balance of microorganisms that keep our skin healthy often gets disrupted, allowing S. aureus to thrive, according to the researchers.

We show that these things can be decoupled, that you don’t necessarily have to have inflammation for the microbe to cause itch, but that the itch exacerbates inflammation on the skin,” said the study’s lead author, Liwen Deng.

The microbe itself initiates a molecular chain reaction, culminating in the urge to scratch. Out of ten enzymes released by S. aureus on the skin, the research team identified one that activates a specific protein in the skin nerve cells. This protein can transmit various signals—touch, heat, pain, and itch—from the skin to the brain.

Itch-Scratch Cycle

Mice exposed to S. aureus developed increasingly intense itching over several days, and repeated scratching led to worsening skin damage. Additionally, the mice became hypersensitive to harmless stimuli that would not normally induce itching. On the other hand, the researchers demonstrated that an anticoagulant medication blocked the activation of the protein, interrupting the itch-scratch cycle. When the experiments were repeated in laboratory dishes with human neurons, they also responded to the specific enzyme released by S. aureus.

We’ve identified an entirely novel mechanism behind itch,” said the study’s senior author, Isaac Chiu, Associate Professor of Immunology at the Blavatnik Institute of the Harvard Medical School. The microbe itself “can be caused by the microbe itself.” The results could help make medicines and creams that can help people who have atopic dermatitis, prurigo nodularis, or psoriasis who suffer from itching that won’t go away. These conditions are linked to an imbalance in the skin microbiome.

Evolutionary Sense

In the future, the researchers aim to investigate whether, in addition to S. aureus, other microbes can induce itching. “We know that many microbes, including fungi, viruses, and bacteria, are accompanied by itch but how they cause itch is not clear,” says Chiu.