Scar tissue is primarily composed of collagen, which is produced by fibroblasts in areas of the skin near the site of the wound. Typically the collagen does not exceed beyond the injured area of skin. However, some people form keloids when their skin is wounded, though scientists do not fully understand why this occurs. A keloid is a type of swollen scar that grows much larger than other scars, appearing similar to a fibrous tumor. Keloids are not especially dangerous, but they may cause discomfort and itching and may limit movement if they become too large, especially if they are located around the joints of the body. In some cases, they may be severely disfiguring as well.