Ultraviolet light and dendritic cells.

The ultraviolet (UV) spectrum is divided into UVC (200-280 nm), UVB (280-320 nm) and UVA (320-400 nm). Of these only UVB and UVA are of environmental significance since UVC is effectively absorbed by ozone in the earth's atmosphere. UVB wavelengths penetrate the epidermis and are almost completely absorbed in the upper dermis while UVA penetrates to the deep dermis.