Dermatography urticaria 

It is a skin disease which known as 'skin writing disease'. This condition is cause by the weak cell membrane of the abundant amount of mast cell (can be recognize as white blood cell and have same function as basophil) in the skin. Mast cell contains histamines which responsible for the swelling of the skin. Only 4% of the world's population have this disease. But I prefer to call this disease as an 'allergic reaction to pressure.'

"Hey Mark! Why do you have scratches on your arm?"
"Well, I scratched my itchy arm like 5 minutes ago and it's swollen. It is called dermatography urticaria. But don't worry. It'll be gone."

Dermatography urticaria by Asupajia_18 September 5, 2015