Blisters are small raised areas that are filled with fluid and located in the superficial layer of the skin. They look like bubbles on the surface of the skin. Although they are often caused by irritation or friction (such as with a poorly fitting shoe), blisters can also represent disease processes. Blisters can accompany some types of skin rashes and inflammatory conditions, including certain autoimmune diseases. Depending upon the cause of the blisters, blisters may occur singly or in groups. In contrast to abscesses and boils, which are collections of inflammatory fluid found deep in the tissues, blisters are found in the most superficial layer of skin. Small fluid-filled blisters are medically known as vesicles. Bullae (singular = bulla) are blisters that are larger than 1 cm across.