<P> A blister is a small pocket of body fluid (lymph, serum, plasma, blood, or pus) within the upper layers of the skin, typically caused by forceful rubbing (friction), burning, freezing, chemical exposure or infection . Most blisters are filled with a clear fluid, either serum or plasma . However, blisters can be filled with blood (known as "blood blisters") or with pus (for instance, if they become infected). </P> <P> The word "blister" entered English in the 14th century . It came from the Middle Dutch "bluyster" and was a modification of the Old French "blostre", which meant a leprous nodule--a rise in the skin due to leprosy . In dermatology today, the words vesicle and bulla refer to blisters of smaller or greater size, respectively . </P> <P> To heal properly, a blister should not be popped unless medically necessary . If popped, the excess skin should not be removed because the skin underneath needs that top layer to heal properly . </P> <P> A blister may form when the skin has been damaged by friction or rubbing, heat, cold or chemical exposure . Fluid collects between the epidermis--the upper layer of the skin--and the layers below . This fluid cushions the tissue underneath, protecting it from further damage and allowing it to heal . </P>

Where does the liquid in a blister come from