Alopecia areata is a condition that causes hair to fall out in small patches, which can be unnoticeable. These patches may connect, however, and then become noticeable. The condition develops when the immune system attacks the hair follicles, resulting in hair loss.

Current evidence suggests that alopecia areata is caused by an abnormality in the immune system that damages hair follicles. This particular abnormality leads to autoimmunity, a misguided immune system that tends to attack its own body. As a result, the immune system attacks particular tissues of the body.