Alopecia areata is a skin disorder that causes hair loss, usually in patches, most often on the scalp. Usually, the bald patches appear suddenly and affect only a limited area. The hair grows back within 12 months or less. For some people, however, the problem can last longer and be more severe, causing total baldness (alopecia totalis) or total loss of body hair (alopecia universalis).
The cause of alopecia areata is probably an autoimmune reaction. This means the body's immune system incorrectly attacks the body's own cells. In the case of alopecia areata, the cells under attack are in the hair follicles (structures that grow hair), especially follicles within the scalp.
Genetic (inherited) factors may play a role, too, particularly when the disorder strikes those under age 30. Almost 40% of people younger than age 30 with alopecia areata have at least one family member who has been diagnosed with the same disorder.
The risk of developing alopecia areata is unusually high in people who have asthma, hay fever, thyroid disease, vitiligo (a condition in which patches of skin lose their color), pernicious anemia and Down syndrome.
Although experts once believed episodes of alopecia areata could be triggered by stress, newer research has failed to prove that stress is a factor.
About 60% of people with alopecia areata experience the first episode of hair loss before age 20. It is usually followed by hair regrowth. However, it's common for the problem to come back. New bald patches can develop at the same time older ones are regrowing hair.