Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Alopecia areata ?
    ### Response:
    Alopecia areata (AA) is an autoimmune disease in which the immune system mistakenly attacks the hair follicles. In most cases, hair falls out in small, round patches on the scalp. Although uncommon, hair loss can be more extensive in some people and affect other parts of the body.
    