What causes alopecia?i just found a bald spot in the back of my head thats the size of a half dollar.my brother and neice have alopecia.

Alopecia areata: Alopecia areata is an autoimmune disease. Autoimmune means that the body's immune system attacks the body. When alopecia areata develops, the body attacks its own hair follicles. A person's genetic makeup, combined with other factors, triggers this form of hair loss. People can have this type of hair loss at any age. Some patients with alopecia areata have a family member who also has the disease.