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, eyelashes, or body hair, but in severe cases, all of the 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. There is an increased incidence of alopecia areata in patients with Down syndrome as well as those with autoimmune diseases, most commonly thyroid disease.

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.

A 2022 retrospective population-based study in Taiwan showed an increase in retinal diseases among 9909 patients with alopecia areata. The disease extent and prior steroid treatment were not commented on; further studies are needed to confirm an association.