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Why do second-degree burns form blisters?

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Cross-section diagram of human skin showing burn depth by degree, with arrows indicating that first-degree burns affect only the epidermis, second-degree burns reach the dermis, third-degree burns extend into subcutaneous tissue, and fourth-degree burns penetrate to muscle. — burn This diagram shows how burn injuries are classified by severity. First-degree burns damage the outer skin, known as the epidermis. First-degree burns are the least severe form of burn injury. Second-degree burns damage the outer skin and some of the underlying skin, known as the dermis, which also includes some connective tissue. Third-degree burns damage all the layers of skin, including subcutaneous tissue. Fourth-degree burns, which are the most severe, destroy every layer of skin and also damage muscle, tendons, and bones.

Second-degree burns form blisters because the injury causes damage that extends through the entire epidermis and into the dermis. This trauma leads to a separation between the layers of the skin—either within the epidermis or between the epidermis and the dermis—creating a space that fills with fluid.12

In the context of a second-degree burn, these blisters are accompanied by redness, pain, and edema, which is the swelling caused by the accumulation of fluids in the wounded tissue. The depth of the burn directly influences the prevalence of these blisters, which often increase in size during the hours immediately following the injury. While blisters are a hallmark of second-degree injuries, they do not occur in first-degree burns, where damage is limited to the epidermis and swelling is minimal.31