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Tuesday, July 18, 2006

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The function of itching and scratching

For quite a long time I have wondered about the function of itching and scratching. Here is some discussion on the topic involving some of my thoughts, from the old Ethology discussion group on the Internet.

I have an article from a book called "Imponderables", which asks the question "Why do we itch?" and replies that the short answer is "we don't know". This book was written in the 1980s and it reports that the same receptors and nerves carry both itching sensations (when they operate at a low level of activity) and pain. However, it now appears that when itching is caused by histamine, specific nerve fibres for itching are involved.

I recently came across a paper by Choi and Maibach (Skin Pharmacology and Applied Skin Physiology 2003; 16: 271-282), entitled "Topical vaccination of DNA antigens: topical delivery of DNA antigens". This paper noted that when skin is damaged, keratinocytes and Langerhans cells become activated. Disruption of the skin barrier results in epidermal Langerhans cell activation as vigorous antigen presenters for T helper cells (that is, an immune response). The level of skin damage required is quite minor. "Stripping" the skin using tape or the removal of dried glue from the skin is sufficient.

Stripping the skin also enhances the effects of topical DNA vaccine. That is, topical application of DNA vaccines with stripping induces stronger immune responses than without stripping.

In conclusion, it seems likely that a key function of itching in humans is to provoke scratching of the skin, with the resulting mechanical disruption leading to an enhanced local immune response. Presumably this would be of value in helping the body to deal with any foreign proteins that had found their way into the skin, including pathogens such as bacteria and parasites.


Julian

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