Canker sores are white because of a protective protein called fibrin that forms as part of the body's natural healing process. When the tissue inside the mouth is injured, white blood cells produce fibrin, which accumulates in the sore, creating a white or yellowish coating. This fibrin membrane acts like a type of scar tissue, protecting the sore while it heals. The white color is a sign that the body is actively healing the canker sore, and it usually appears in the later stages of healing as the sore closes over and the pain starts to lessen. Dead tissue and cellular debris (called slough) can also contribute to the white appearance, but the fibrin membrane is the primary cause.
