“There are numerous theories about it, but it’s generally agreed upon that a wound itches when histamine is released by the cells in response to physical trauma and the introduction of some type of bacteria. Histamine is a substance produced by the body that can cause an allergic type of reaction when it is released; this explains the itchy skin one might experience when dealing with a wound.
Just how the process is triggered is a more scientific explanation. When the skin’s nerve endings are stimulated enough to itch, the sensation is transmitted to the body’s C-fibers. The fibers, in turn, respond to the sensation by essentially “delivering” the itch to the skin surrounding the nerve endings. The result is an itchy, uncomfortable wound. ”
Source