Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Langerhans cell histiocytosis ?
    ### Response:
    Langerhans cell histiocytosis is a disorder in which excess immune system cells called Langerhans cells build up in the body. Langerhans cells, which help regulate the immune system, are normally found throughout the body, especially in the skin, lymph nodes, spleen, lungs, liver, and bone marrow.
    