Alopecia areata (AA) is probably the third most common form of hair loss dermatologists see, after androgenetic alopecia and telogen effluvium. The lifetime risk for AA is nearly 2%, or two in every 100 people will get AA at some point in their lives. It is not contagious; you can't catch AA from someone who has it.
Researchers believe AA is an autoimmune disease such as rheumatoid arthritis, but in this case the individual's own immune system attacks hair follicles instead of bone joints. Just why or how AA develops is not clear. For whatever reason, the immune system is inappropriately activated and attacks hair follicles. Research using several disease models shows certain types of lymphocytes play a primary role in the hair loss. They seem to attack the hair follicles, mistakenly thinking that somehow they are a threat to the rest of the body.
AA can affect men, women, and children. It often appears as well-defined circular bald patches on the scalp. Many people will get just one or two patches, but for some the hair loss can be extensive. Unfortunately, children who develop AA before puberty are most likely to develop more extensive and persistent hair loss.
Hair loss that spreads to cover the entire scalp is called alopecia totalis. If it spreads over the entire body, affecting scalp, eyebrows, lashes, beard, pubic hair, and everything else, then the condition is called alopecia universalis. If the alopecia is just limited to the beard area in men, it is called alopecia barbae.
The inflammation involved in AA focuses on the roots of hair follicles deep in the skin. As a result there is very little visible at the skin surface. There is no redness and often no pain, although a few people do find their skin itchy or painful to touch in the very early stages of AA development. Usually, though, there is no sensation -- just a patchy shedding of hair.
The hair loss can be quite sudden, developing in a matter of a few days and it may happen anywhere on the scalp. The patch is usually smooth bald skin with nothing obvious to see beyond the absence of hair. Unlike other autoimmune diseases, the target of the inflammatory response in AA, the hair follicles, are not completely destroyed and can re-grow if the inflammation subsides.