An inflammatory condition of the skin and mucous membranes. Lichen planus occurs when the immune system mistakenly attacks cells of the skin or mucous membranes. On skin, lichen planus appears as purplish, itchy, flat-topped bumps. On mucous membranes, such as in the mouth, it forms lacy, white patches, sometimes with painful sores. Lichen planus usually goes away on its own. If symptoms are bothersome, topical creams and oral antihistamines may help.