How Itches Work

Almost all animals — and people — itch. But why?
Almost all animals — and people — itch. But why?
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Usually you itch, then you scratch. Then maybe you itch and scratch some more. Eventually, the flare-ups of itching subside. But for one woman in Massachusetts, the itching and scratching cycle didn't stop.

It started right after an episode of shingles and the constant itching on her head took over her life for years. No medication or treatment did anything to solve it. While she could sometimes manage to control the scratching during the day, she couldn't stop her hands from migrating to her head at night, tearing away at her skin. One morning, she woke up to a greenish fluid trickling down her face. She'd scratched through her skull and into her brain. You read that right: her brain. She recovered from that episode and after a few years of sleeping under restraints, taught her body not to scratch at night. But the itch was still there (as of 2008), and what causes it remains a mystery [source: Gawande].

In general, itching (also called pruritis) is a bit of a mystery because we don't know exactly why it happens. It's the sensation arising from the irritation of skin cells or nerve cells associated with the skin. Itching plays an important role in keeping us safe, as it can be a trigger to alert us to potential harm, such as a spider crawling on our leg.

Many things can cause us to itch: dermatological conditions, allergic reactions, even different types of psychosis. A treatment to help one type of itch may do nothing to alleviate another. For a long time, itching was thought to be a lesser version of pain, so scientists and doctors tended to focus their research on looking for a connection. While there's still some evidence to show the two sensations are close cousins, new studies show that itching is a beast of its own. We know very little about the mechanism of itching — how the skin talks to the brain and creates the itch.