Skin constitutes a very peculiar tissue with outstanding physical and physiological properties. Recent findings of the past few decades place new emphases on the role of skin in the immune system. Indeed, immunity has developed within cutaneous environment to a specialized and specific system. These features were highlighted by Streilein (1) with the concept of skin-associated lymphoid tissues (SALT). Lymph nodes draining the skin are integral components of the SALT due to their role in providing a site for interaction of immunocompetent lymphocytes and antigen-presenting cells.

Once the skin is disrupted (trauma, UVB irradiation, pathogenic microorganisms), an array of both resident and infiltrating cells contribute to cutaneous immunity including Langerhans cells (LCs), keratinocytes (KCs), dendritic epidermal T lymphocytes (DETC), γδ T cells, mast cells, tissue macrophages, granulocytes and endothelial cells. Foreign material introduced into or on the skin could be trapped by LCs residing in the basic layer of proliferating KCs. LCs are considered as «sentinels» capturing and processing antigens. As a consequence of inflammation, LCs mature and express molecules that promote their migration from the epidermis to the draining lymph nodes via the lymphatic vessels and lead to stimulation of naive T lymphocytes in T-eell areas of lymphoid tissues. The immunological interactions between KCs, DETC and LCs are orchestrated by soluble mediators, particularly cytokines. Keratinocytes have been identified as a major source for several cytokines such as Tumor Necrosis Factor-alpha (TNF-α), involved in the migration of DCs, or interleukin-7 (IL-7) acting as a growth factor for T cells.