The cutaneous citadel: a holistic view of skin and immunity.

Human skin has 4 major functions: endogenous homeostasis (e.g. regulation of body temperature and fluid balance), metabolism (e.g. Vitamin D synthesis), sensory input, and to serve as a barrier to external threats (e.g. infection, mechanical injury, ultraviolet light). It is the latter function which concerns this review, for the skin's remarkable success in protecting the human body from the outside world is a major component of our immune system. The eminent pathologist, Virchow, whose work in the mid 19th century revolutionized many aspects of medical understanding, viewed the skin as an effective but inanimate barrier (1). However, recent technologies have elucidated a highly complex, dynamic interplay between the skin and other members of the immune system.