A blister (bulla) is a bubble of fluid that forms beneath a thin layer of dead skin. The fluid is a mixture of water and proteins that oozes from injured tissue. Blisters most commonly form in response to a specific injury, such as a burn or irritation, and usually involve only the topmost layers of skin. These blisters heal quickly, usually without leaving a scar. Blisters that develop as part of a systemic (bodywide) disease may start in the deeper layers of the skin and cover widespread areas. These blisters heal more slowly and may leave scars.

Many diseases and injuries can cause blistering, but three autoimmune diseases—bullous pemphigoid, dermatitis herpetiformis, and pemphigus vulgaris—are among the most serious. In an autoimmune disease, the body's immune system, which normally protects the body against foreign invaders, mistakenly attacks the body's own cells (see Allergic Reactions and Other Hypersensitivity Disorders: Autoimmune Disorders)—in this case, the skin.

Last full review/revision September 2008 by Julie E. Russak, MD