Ariana Page Russell has a skin condition called dermatographia, where the skin is overly sensitive to minor injuries. Even light scratches will cause it to become red and raised.

Ever the artist, Ariana decided to use her medical condition as the basis for her art:

My own skin frequently blushes and swells. I have dermatographia, a condition in which one’s immune system exhibits hypersensitivity, via skin, that releases excessive amounts of histamine, causing capillaries to dilate and welts to appear (lasting about thirty minutes) when the skin’s surface is lightly scratched. This allows me to painlessly draw patterns and words on my skin, which I then photograph.

Some samples:

Link: Ariana's website - via It's Nice That and about:blank