Hair is lost because it is rejected by the affected person’s immune system which does not recognise the hair follicles as "self", but regards them as "foreign" (autoimmunity). Why this happens is not fully understood, nor is it known why only localised areas are affected and why the hair regrows again.
Someone with alopecia areata is more likely than a person without it to develop other autoimmune conditions such as thyroid disease, diabetes and vitiligo (white patches on the skin), although the risk of getting these disorders is still low. Your doctor may suggest a blood test looking for antibodies that may predict whether you are likely to develop thyroid problems or pernicious anemia.
Alopecia areata is not catching nor is it related to diet or vitamin deficiencies. Stress, particularly events such as bereavement, separation and accidents, occasionally appears to be a trigger for alopecia areata.
There is a genetic predisposition to alopecia areata and close family members can be affected. Thyroid problems or diabetes are also more common.