Dr. Maya Jerath, the director of the Allergy and Immunology Clinic at UNC-Chapel Hill School of Medicine, explains one of the biggest annoyances known to man - itching. Questions and answers have been edited.
Q: Why do we itch? I have heard the theory that the same nerve fibers that send pain signals also send itching signals, meaning that itching is actually a form of light pain. Is that true?
Itching is a very frustrating condition - both for the patient and the physician - and is quite poorly understood. There are special fibers that conduct itching signals, and these are distinct from pain fibers. These nerves conduct slowly and pick up signals from quite some distance away. Interestingly, these fibers were only fairly recently discovered, in the last 15 years or so. The signals go from the periphery to the spinal cord then to brain.
The fibers fire in response to chemical signals as well as physical signals. An example of a chemical signal is the one provided by histamine. Histamine is a chemical that is released by cells in an allergic reaction, hence the itch associated with allergies. An example of a physical sensation would be itching triggered by feeling something brushing against you. Some people believe that the itching sensation and the scratch response evolved to protect us from bugs and poisons.
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Q: Why do we seem to feel itchier in the winter?
We itch more in winter because our skin gets dry in the cold weather, and this heightens itchy sensations.
Q: Why do some people itch more than others? Some people get eczema whereas others don't; some people are allergic to poison ivy while others aren't; some people have to slather on the lotion in the winter, whereas others survive the season itch-free.
I think the basic fact is that everyone's body is different. The differences we see in the way people get eczema or react to poison ivy have to do with differences in people's immune systems, which is partially driven by genetics and probably by differences in environmental exposure. And some people are drier than others simply because of differences in their skin. Of course, skin does tend to dry with age, so that is a factor. The care one takes of the skin is also a factor; frequent washing, very hot water, etc., are drying.
Q: Does it help or hurt to scratch an itch?
In terms of whether scratching an itch hurts or helps - well, it does set up an itch-scratch cycle, and excessive scratching can cause excoriations of the skin (skin breakdown) and is believed to make the rash of eczema worse. But it sure is true that there are few things as wonderful as the sensation of scratching an itch.