scratch it, it's biologically programmed

Today I learned about itching. Apparently also known as pruritis. It's pretty simple actually, itching is simply a response by your epidermis to any various type of irritant. Impulses are sent from your skin to your brain with the impulse to scratch. You scratch to remove the irritant.
The chemical messenger active in an itch is Histamine which is released by the mast cells in your skin. It's simply another evolutionarily developed method for our bodies taking care of themselves. Now I know.

read:
How Stuff Works: Itching
Wikipedia: Itching