It is an inflammatory condition that can affect skin and mucous membranes. On the skin, lichen planus usually appears as purplish, often itchy, flat-topped bumps. On mouth, vagina and other areas covered by a mucous membrane, lichen planus forms lacy white patches, sometimes with painful sores. Lichen planus occurs when immune system mistakenly attacks skin cells or mucous membranes. The reason for this abnormal immune response is unknown. You can't catch lichen planus or give it to another person. Most people can manage typical, mild cases of lichen planus at home, without prescribed medical treatment. If the condition causes pain or significant itching, one may need medication to suppress immune system. The symptoms of lichen planus vary depending on the areas affected. Typical signs and symptoms include purplish, flat-topped bumps, most often on the inner forearm, wrist or ankle, but sometimes on the external genitals.