White dermatographism is a physical sign which is classically associated with atopic dermatitis. It is considered to be indicative of high levels of circulating IgE, which cause a more vigorous reaction than usual dermatographism, which is red. Dermatographism is differentiated from post-traumatic erythema by being a more exaggerated response to a much less dramatic stimulus. Think of it as physically-induced localized urticaria. There are other conditions in which white dermatographism has been reported, but the money is on atopic dermatitis. Incidentally, there is a condition known as "black dermatographism" which is not a true urticarial response, but rather is a staining underneath a ring or wristband of a watch with subsequent linear discoloration of the skin.