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What Causes Alopecia Areata?
In alopecia areata, immune system cells called white blood cells attack the rapidly growing cells in the hair follicles that make the hair. The affected hair follicles become small and drastically slow down hair production. Fortunately, the stem cells that continually supply the follicle with new cells do not seem to be targeted. So the follicle always has the potential to regrow hair.

Scientists do not know exactly why the hair follicles undergo these changes, but they suspect that a combination of genes may predispose some people to the disease. In those who are genetically predisposed, some type of trigger--perhaps a virus or something in the person's environment--brings on the attack against the hair follicles. - NIAMS