Whether caused by injury, surgery, or burn, scars can be disfiguring. Scars stand out against the rest of the skin because scar tissue is made of collagen cells rather than ordinary skin cells. As a result, scars are usually a different color, and do not have sweat glands or hair follicles. The severity of a scar depends on many factors, including the size and depth of the wound, the blood supply to the area, and the thickness and color of the skin. Some people – especially those with deeper skin tones – have a tendency to produce prominent, raised scars called keloids. The success of scar revision often depends on the type of scar.