The epidermal Langerhans cell is the bone marrow-derived dendritic, antigen-presenting cell of the skin. It is characterised by a unique intracytoplasmic organelle--the Birbeck granule--and constitutively expresses class II MHC molecules and the CD1a glycoprotein. The Langerhans cell represents one of the most potent antigen-presenting cells of the body, and fulfils an important role in detecting foreign antigen entering the body through the skin and in immune surveillance. The distribution of Langerhans cells is restricted to the skin, lymph nodes, bronchial mucosa and thymus. The discovery by Nézelof in 1973 that the lesional cells in the disease then called 'Histiocytosis X' contained Birbeck granules established the close relationship between the Langerhans cell and this disease and led ultimately to the adoption of the name Langerhans cell histiocytosis to replace the older term. The LCH cell expresses the phenotype of a Langerhans cell apparently 'fixed' at an early stage of cell activation. The LCH cell is, however, functionally defective in antigen presentation, and the tissue distribution of the disease--affecting bone, skin, lymph node, lung, liver, spleen, CNS, gastro-intestinal tract and bone marrow--is quite different from the normal distribution of the Langerhans cell. Studies are now under way throughout the world to investigate the relationship between the normal Langerhans cell and the LCH cell. Specifically we need to identify whether the LCH cell is a cell arrested at a specific time in normal Langerhans cell ontogeny or if it represents a response to a biological insult to the mature Langerhans cell or its precursors.