Why do people itch?

-- Stephanie Patterson, 12, Columbia

Whoever came up with the word "itch" sure did a good job. It's the perfect word to describe that weird sensation that makes your brain scream "Scratch!" Experts say that just hearing the word "itch" can cause some people to, well, itch.

Itching -- which doctors call "pruritus" -- is pretty much a universal experience. Dry skin, poison ivy, mosquito bites, mites and other parasites are just some of the things that make humans itch. Alan Fleischer, a dermatologist (that's a skin doctor) and itch expert at Wake Forest University Baptist Medical Center in North Carolina, says that the last time he went to the zoo, he noticed that every animal spent time scratching. Even fish brush up against underwater rocks and logs to relieve their itchy scales.

Itches may be everywhere, but nobody's quite sure exactly how they work. Here's what we do know: Human skin contains all kinds of nerves, each of which carries different kinds of messages to the spinal cord, which in turn sends those messages to the brain.

The brain makes a quick decision about how to respond: If the message comes from a nerve whose job it is to report pain or heat, the brain will shout "Run away!" and you will withdraw whatever body part has been exposed to the source of pain or heat. If someone gently rubs your arm, a different nerve will tell your brain about it, and your brain will answer, "Aaah, that feels good. More, please."

Some of those nerves are there to report itches. Fleischer says that scientists are only now starting to understand how the brain processes itch messages. But one thing is for sure: When your brain hears there's an itch around, it doesn't waste any time telling you to scratch.

Scratching might be the body's way of distracting itself from the unpleasant itchy feeling by creating a feeling of mild pain, which takes your mind off the itch. Fleischer says that there may be some useful purpose in scratching, too, as it might help people and other animals locate ticks or other parasites (creatures that attach themselves to other creatures) so they can remove them.

It's very hard to resist the urge to scratch. (Try it sometime, and see how long you last.) But if an itch hangs around for a while, as it does when you have poison ivy, a mosquito bite, a rash or head lice, rather than just keep scratching, you need to figure out what's causing the itch and do something about it -- maybe apply an ointment.

Constant scratching tears your skin and leaves you open to infection. And that is likely to cause even more itching. That's what doctors call the "itch-scratch cycle." You don't want to get caught up in it.

-- Jennifer Huget

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We know that animals also have an itch to scratch.