Alopecia areata is a T-lymphocyte-mediated autoimmune disease of the hair follicle resulting in nonscarring hair loss. The condition can be limited to a few small patches of alopecia that involve the scalp, eyebrows, eyelashes, or body hair (patchy alopecia areata), 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 occurs in individuals of all ages and races / ethnicities; however, it is most commonly seen in people younger than 40 years (mean age of onset is 25-36 years). For alopecia areata that presents at an early age (eg, early-onset alopecia areata, which has a mean onset between ages 5 and 10 years), there is typically a more severe presentation (eg, alopecia universalis). There is an increased incidence of alopecia areata in individuals 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. Alopecia areata following COVID-19 infection and COVID-19 vaccination has also been reported.

There is a complex interplay of genetic and environmental factors that play a role in the pathogenesis of alopecia areata. 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.