blister

blister puffy swelling of the outer skin (epidermis) caused by burn, friction, or irritants like poison ivy. A response of the body to protect deeper tissue, blisters generally contain serum, the liquid component of blood. The so-called blood blister, however, forms over ruptured capillaries and therefore contains whole blood.

blister A raised, well circumscribed lesion of the skin, containing a sterile fluid, derived from the serum. Strictly, a blister or bulla has a diameter of greater than 5 mm, smaller lesions being called vesicles. Commonly they become infected and fill with pus, and are then known as pustules. They are caused by trauma (friction, burns, and scalds), allergic contact dermatitis, insect bites, sunburn, etc. In earlier times agents were used to raise vesicles or blisters on the skin to relieve the pain from deeper structures by the process of counter-irritation. A commonly used vesicant was a powder derived from the pulverized dried beetles of Lytta vesicatoria, containing cantharidin, and commonly known as Spanish Fly or Blistering Beetle.

Alan W. Cuthbert