The sudden loss of a piece of hair on the back of the head is often considered to be caused by pemphigus vulgaris, which is a kind of sudden and limited patchy alopecia, and its causes include heredity, immunity, and neuropsychiatry. 1. Genetic factors: baldness has a certain hereditary tendency, if the parents have baldness, the children may have baldness symptoms. 2. Immune factors: If the body has an immune response against the hair follicle’s own antigen, or if the immune immunity mechanism of the hair follicle is impaired during the growth period, it may lead to pemphigus vulgaris. In addition, if you suffer from diabetes, vitiligo and other autoimmune diseases, it may also induce or aggravate baldness. 3. Neuropsychiatric factors: if long-term mental tension, anxiety and depression, excessive pressure, etc., may induce baldness. There is no single cause of baldness, and patients need to have a regular daily routine and maintain a good mindset. It is recommended that patients go to a regular hospital for consultation to clarify the cause of the disease.