Blisters tend to form whenever a deeper skin layer is damaged and the surface skin remains intact. As the body sends serum and other fluids to the injury site, a liquid-filled pocket often forms. This pocket is commonly referred to as a blister. Blisters are not the same as callouses, which tend to form over skin abrasions and are not filled with liquid. The most common causes of blisters are burns, abrasions and foreign objects under the skin. Their formation is usually not the problem in a first aid situation- blisters are a natural reaction to an injury below the skin's surface. The real problem with blisters is protecting the injured area from further infection or damage. Treating a blister on the hands essentially means keeping it clean and intact until it can heal naturally.