The skin as an organ of immunity.

During evolution, the skin has developed a specific immunological environment that is known as the skin immune system (SIS). A substantial number of immunological phenomena exemplify the special place the skin occupies as a peripheral immune organ. These include the continuous exposure to sun rays, with subsequent immunological consequences, an area of investigation known as photoimmunology. Also special is the presence of a dense network of dendritic antigen-presenting cells (Langerhans cells) in the epidermis, which are functionally related to the skin-specific phenomenon of the contact allergy type of delayed hypersensitivity. The acronym SIS is designed to give a complete picture of the skin's functions as an immunological organ, and includes the humoral systems in addition to the cellular subsets taking part in it. In this article, the history of the development of immunological concepts of skin is exemplified with some recently explored areas of immunodermatological research.