Rare condition allows woman to draw on her skin without ink

Skin of people with Dermatographia

How 27-year-old Louise Sara from United Kingdom, write the words under her skin like a notepad?  This condition is called dermatographia or skin writing. When people who have dermatographia a light scratches to their skin will result the scratches to redden into a raised wheal.

Dermatographia is thought to be caused when the cells under the surface of the skin release histamines under the slightest pressure.

Histamines are chemicals released in the body as part of an allergic reaction, causing the skin to swell.

Louise is not having any allergic reaction is just a mechanical trauma that leads to the release of histamines,  this is the immune system's response causing this leakage and extreme increase in blood flow to that area to try to correct the damage.

Source: YouTube by Science Channel