In alopecia areata, the affected hair follicles are mistakenly attacked by a person's own immune system (white blood cells), resulting in the arrest of the hair growth stage. Alopecia areata usually starts with one or more small, round, smooth bald patches on the scalp and can progress to total scalp hair loss (alopecia totalis) or complete body hair loss (alopecia universalis).

http://www.jci.org/cgi/reprint/101/1/62.pdf