Alopecia areata is a T-lymphocyte mediated autoimmune disease of the hair follicle resulting in nonscarring hair loss. Most cases are limited to 1 or 2 small patches of alopecia that involve the scalp, eyebrows, or body hair, but in severe cases, all hair on the scalp is lost (alopecia totalis) or all scalp and body hair is lost (alopecia universalis). History of sudden onset is characteristic.

Alopecia areata is seen equally in both sexes and in patients of all ages and ethnicities; however, it is most commonly seen in patients between the ages of 5 and 40. There is an increased incidence of alopecia areata in patients with Down syndrome as well as those with autoimmune diseases, most commonly thyroid disease. In addition, there may be a family history of alopecia areata or autoimmune disease. Patients with alopecia areata are also more likely to have atopy, and its presence is felt to be a poor prognostic indicator.

The course of alopecia areata is unpredictable, with wide variation in duration and extent of disease occurring from patient to patient. In most patients, hair will eventually spontaneously regrow, although recurrences are common. The condition is treatable but cannot be cured.