The liquid inside a blister is called serum, which is a clear, watery fluid that consists of water, protein, and carbs that come out of leaky or injured blood vessels. Serum cushions and protects the underlying tissue from damage so that it can heal, and if the blister remains unopened, serum can provide natural protection for the skin beneath it. Most blisters are filled with clear fluid, either serum or plasma, but they can also be filled with blood (known as "blood blisters") or pus if they become inflamed or infected.