Skin, immunity and the nervous system.

Connections between nerve fibres and cutaneous cells have been studied using confocal and electron microscopy. In the skin, nerve fibres may secrete neuromediators, i.e. substance P, vasoactive intestinal peptide, somatostatin, calcitonin-gene-related peptide, gastrin-releasing peptide, neuropeptide Y, peptide histidine-isoleucine, neurotensin, neurokinins A and B, bradykinin, acetylcholine, catecholamines, endorphins and enkephalins. Neurohormones such as prolactin, melanocyte-stimulating hormone and adrenocorticotrophic hormone are also expressed in the skin. Neuromediators and neurohormones may be secreted by cutaneous cells, which also express receptors. Functions of epidermal and dermal cells are modulated by these substances. Immune cells transiently present in the skin (e.g. macrophages and lymphocytes) are modulated by neuromediators through receptors. During the course of skin disorders, especially inflammatory reactions, the neuroimmunocutaneous system is destabilized. This is particularly true in psoriasis. This destabilization may be secondary, although evidence shows it can also be responsible for the induction and maintenance of the inflammatory process. The skin, the nervous system and immunity are not independent systems but are closely associated and use the same language of cytokines and neurotransmitters. A new concept is suggested: the neuroimmunocutaneous system.