A blister is a small pocket of fluid 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 called serum or plasma. However, blisters can be filled with blood (known as blood blisters) or with pus (if they become infected).

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.

Causes

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.

Friction or Rubbing

Picture showing a relatively large blister, the dead skin has been pushed down to expose the flesh underneath. Intense rubbing can cause a blister, as can any friction on the skin if continued long enough. This kind of blister is most common after wearing a new pair of shoes.PFD. "The Skin Surface and Friction," British Journal of Dermatology. 1955;67:239-248.PFD. "Experimental Friction Blisters," British Journal of Dermatology. 1955;67:327-342.MB, Cortese TA, Fishman L, Wiley HS. "Studies on Blisters Produced by Friction," Journal of Investigative Dermatology. 1966;47:456-465. Blisters are most common on the hands and feet too, as these extremities are susceptible while walking, running, or performing repetitive motions. Blisters form more easily on moist skin than on dry or soaked skin, and are more common in warm conditions. Less aggressive rubbing over long periods of time may cause callus to form rather than a blister. Both blister and callus can lead to more serious complications, such as foot ulceration and infection, particularly when sensation or circulation is impaired, as in the case of diabetes, neuropathy or peripheral artery disease (PAD).

Extreme temperature

First and second degree burns may result in blistered skin. Blisters can also form on the hands and feet as a result of tissue damage incurred by frostbite.

Chemical exposure

Sometimes, the skin will blister when it comes into contact with a cosmetic, detergent, solvent or other chemical. This is known as contact dermatitis. Blisters can also develop as a result of an allergic reaction to an insect bite or sting.

Crushing/Pinching

A blood blister usually forms when a minute blood vessel close to the surface of the skin ruptures (breaks) and blood leaks into a tear between the layers of skin. This can happen if the skin is crushed, pinched or aggressively squeezed.