This article doesn’t answer that question, but it does explain why scratching relieves the itch.  Check it out (via MindHacks).  Apparently scratching turns off activity in nerve cells, but somehow the nerve cells knew you were itchy because they do not shut off in response to a scratch when you are not itchy.

Did you know that just reading about itchiness can make you itchy?  Did you scratch while reading this?

Can you resist an itch until it goes away?  Try it! (Deep breathing seems to make it go away faster.)