It can be caused by allergies, insect bites, skin dryness or any affection.
The American Academy of Allergy, Asthma and Immunology explains that although most itches are just bothering and uncomfortable, an excessive scraping can damage the skin protecting layer and expose the body to germs and infections.
An itching is frequently unchained by the histamine (a hormone produced by the body during allergic reactions), a chemical associated to immunologic responses, which gives rise to itching and redness in cases such as insect bites, skin dryness or damages.
A specialist doctor is the one who will identify what produced a persistent or recurrent itching and determines the most suitable treatment.
In the following video you can learn in detail what histamine is, and how it acts on the body.