Bullous pemphigoid is an autoimmune disease, which means that the cells in the body that normally fight infection attack the body instead. The body's immune system is confused and makes an antibody (type of protein used to fight infection) that targets a part of the skin that normally holds it together. The attack on the skin causes blisters (firm, fluid-filled bubbles on the skin) to form. This disease most often involves only the skin, but the eyes, mouth, and genitals also can be affected. In most cases, the disease develops on its own, but certain medications also can cause bullous pemphigoid to develop.