I would like to expound in my own words what the good Dr. Tsai has mentioned in his book on why it is a good idea to scratch your skin to get rid of toxins.
In general, doctors will tell you not to scratch your skin when you suffer from an itching sensation. This is wrong because itching is the result of your bodily processes ridding allergens (foreign bodies) from the body through the skin. There is only one way to scatter the foreign matter out from the body: to scratch. Therefore, scratching is a necessary and sufficient action for eliminating allergens out of the body.
I was gifted with a fantastic book by Dr. Tsai, the Chinese doctor educated in Japan and one who has clinics worldwide specializing in the cure for all skin diseases. In the following excerpt, Dr. Tsai mentions 3 fundamental mistakes in skin disease treatment. 
