One of my flatmates at university frequently demonstrates to us an amazing phenomenon. If you take a blunt object, such as a chopstick, you can write her name on the skin of her back. Within a few seconds, the name begins to turn red and the skin then swells up wherever something has been written. She can parade around the flat for hours, with her name emblazoned in the skin of her back. When I try the same on my back, nothing happens. Why is her skin so special?

Dr Fred Kavalier answers your health question:

Your flatmate has dermatographism, which is a skin response to physical pressure. The rash that appears is similar to the rash that occurs when you come into contact with nettles. Nettle rash is caused by a chemical contained in the plant. Dermatographism is caused by the pressure of the chopstick on the skin. About 2-5 per cent of the population has dermatographism. It is a form of urticaria, which is a family of conditions that cause skin rashes. People who have urticaria tend to develop raised, itchy red rashes in response to a variety of stimuli. Sometimes the stimulus is chemical, as with nettle rash. But some people will respond in the same way to simple physical pressure on the skin. The redness is caused by the release of the chemical histamine.

Please mail your questions for Dr Fred to He regrets that he is unable to respond personally to questions.