Acne scars are a result of injury to the tissue caused by the body's inflammatory reaction to sebum, bacteria and dead cells that have plugged the follicle. Depressed areas and raised thickened tissue (keloids) are an example of true scars. Some skin marks may appear to be scars, but can actually be "pseudo scars" or "post inflammatory pigmentation." Pseudo scars can appear as reddish spots that can proceed acne, remaining for months. Post-inflammatory pigmentation is characterized by a darkening of the skin at the site of a healed acne lesion that can persist for up to 18 months.