What are Langerhans cells?

Langerhans cells alert components of the adaptive immune system in response to the presence of pathogens and bacteria on the skin. These cells are mononuclear phagocytes which originate from common macrophage precursors in the skin epidermis prior to birth. Langerhans cells are derived from CD34 positive stem cells of bone marrow and exist in the mononuclear phagocytic system. Langerhans cells are first line defenders from substances getting into the skin and also have a significant role in antigen presentation. They show fragments of engulfed pathogens on their cell surface to stimulate an adaptive immune response. These cells express MHC I and MHC II molecules, uptake antigens in the skin and transport to the lymph node.