Langerhans cell histiocytosis, previously known as a rare childhood disease, involves an abnormal proliferation of a type of tissue cell called a histiocyte. The cause is not definitively known, but it likely stems from an immune system disruption. In its mildest form, the disease results in rapid cell growth in a single bone, often the skull, a clavicle, a rib, or a vertebra, leading to swelling and pain. In its most severe and least common form, symptoms include a rash, enlargement of the liver, spleen, and lymph nodes, and lung involvement. Treatment typically involves anticancer drugs, but the prognosis remains grim.