Ontogeny and homeostasis of Langerhans cells.

Langerhans cells (LCs) refer to the dendritic cells (DCs) that populate the epidermis. Strategically located at one of the body's largest interfaces with the external environment, they form the first line of defense against pathogens that breach the skin. Although LCs share several phenotypical and functional features with lymphoid and non-lymphoid organ DCs, they also have unique properties that distinguish them from most DC populations. In this review, we will discuss the key mechanisms that regulate LC homeostasis in quiescent and inflamed skin. We will also discuss recent evidence that suggests that LCs arise from dedicated precursors during early embryonic development.