Alopecia areata is an autoimmune disease that causes hair to fall out in small, random patches. The hair loss usually affects the scalp, but it can also occur in other areas of the body. The main symptom of alopecia areata is hair loss. Hair usually falls out in small round patches on the scalp. These patches are usually several centimeters or less. Hair loss might also occur on other parts of the body. You may first notice clumps of hair on your pillow or in the shower.