Alopecia areata is considered an
autoimmune disease, in which your immune system mistakenly attacks a part of your body. It is unclear, however, how alopecia areata can cause the slowing of hair production.
In alopecia areata, white blood cells attack the rapidly growing cells in the hair follicles that make the hair. The affected hair follicles become small, which drastically slows down hair production.
Although it is not known how the follicle damage from alopecia areata causes hair production to slow, scientists suspect that a combination of genes may predispose some people to this autoimmune disease.
(Click Cause of Alopecia Areata for more information on alopecia areata and hair loss, the function of the immune system, and the theories associated with genetics.)