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
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.