Alopecia areata is a T-lymphocyte-mediated autoimmune disease of the hair follicle resulting in nonscarring hair loss. It is the second most common type of nonscarring hair loss, following male and female pattern alopecia. Most cases are 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 patients of all ages, races, 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. Patients with alopecia areata are also more likely to have atopy, and its presence is felt to be a poor prognostic indicator.

There is a complex interplay of genetic and environmental factors that play a role in the pathogenesis of alopecia areata. While medication-induced alopecia areata has been uncommon in the literature, published articles report an association with some medications, including TNF inhibitors (eg, adalimumab, etanercept, and infliximab). More recently described culprit agents include proton pump inhibitors and immune checkpoint inhibitors (eg, anti-CTLA-4 and anti-PD-1) used to treat malignancies. Alopecia areata following COVID-19 infection and vaccination has also been reported.

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.