Rounded skin elevation in which fluid fills a separation between layers of epidermis or between the epidermis and the dermis. The fluid is usually clear; yellowish fluid contains pus, and red fluid contains blood. Blisters often occur on the palms or soles when pressure and friction cause an upper skin layer to move back and forth over the one under it. A small gap opens between them and becomes filled with fluid. This type generally heals spontaneously, sometimes leaving a thickened callus. Blisters that occur as symptoms of contact dermatitis, viral infection, or autoimmune disease can appear anywhere on the body and may leave scars.