Skin as a peripheral lymphoid organ: revisiting the concept of skin-associated lymphoid tissues.

Antigen presentation to T cells is essential for the induction of adaptive immunity. This event takes place not solely in the lymph node (LN) but also in the skin. Recent in vivo trafficking studies using Kaede-transgenic mice reveal that skin-homing effector memory T cells alter their effector function and homing ability by transitioning to a central memory T cell-like phenotype through antigen recognition that occurs in the skin. In addition, these cells travel back and forth between the skin and draining LNs. These studies are evocative of the classic concept of skin-associated lymphoid tissues and underscore the critical role of skin as a peripheral lymphoid organ.