Light micrograph section through a human tissue system showing Langerhans cell histiocytosis (LCH). This 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. Excess immature Langerhans cells usually form tumours called granulomas.