For the episode, see The Itch.
An itch is an unpleasant sensation on the skin that encourages scratching. It's usually caused by irritation and inflammation at the site of the itch, often from an allergic reaction. Unlike pain, itching does not result in a withdrawal or guarding reflex. However, an itch does not necessarily indicate an issue at the site of the itch and may instead be a symptom of issue with the central nervous system, such as a brain tumor, multiple sclerosis, or peripheral neuropathy. It's occasionally a sign of narcotic abuse, and can also be caused by mental illness.
The more prosaic causes of itching are lice, hives, dry skin, skin diseases (like athlete's foot), and the healing of injuries such as scratches and scrapes.
Although many topical preparations are effective against itching, antihistamines and steroids can also be used in severe cases.