Skin Writing

Dermatographic urticaria also known as dermographism, dermographia, contact or pressure urticaria or 'skin writing'. This skin disorder is characterized by eruptions and inflammations on the skin when an external offending stimulus comes in contact with the skin. 

Human skin contains certain cells called mast cells, which protect it against pathogens. The membranes surrounding these cells are weak for some people. In such cases, when the skin is rubbed or scratched, it will release a substance called histamine, which results in the damage of these membranes that causes rashes and swelling of the skin.

In most cases, the actual cause for dermatographic urticaria is unidentified, yet shared by 4-5% of the population.

Info from here, here and here