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A blister or bulla is a defense mechanism of the human body. When the outer (epidermis) layer of the skin separates from the fibre layer (dermis), a pool of lymph and other bodily fluids collect between these layers while the skin re-grows from underneath. Blisters can be caused by chemical or physical injury. An example of chemical injury would be an allergic reaction. Physical injury can be caused by heat, frostbite, or friction.