Radiation dermatitis, burns, and recall phenomena: Meaningful instances of immunocompromised district.

Ionizing and ultraviolet radiations, as well as burns, can selectively damage and immunologically mark the cutaneous area they act on through direct and indirect mechanisms. After the causal event has disappeared, the affected skin district may appear clinically normal, but its immune behavior is often compromised forever. In fact, irradiated or burned skin areas undergo a destabilization of the immune control, which can lead to either a reduction of immunity (as suggested by the facilitated local occurrence of tumors and infections) or an excess of it (as suggested by the possible local onset of disorders with exaggerated immune response). In other words, these areas become typical immunocompromised districts (ICD). Also, in recall phenomena the damaged skin area usually behaves as an ICD with an exaggerated immune response toward a wide range of drugs (especially chemotherapeutic agents) that prove to be harmless on the undamaged skin surface. The occurrence of any skin disorder on an irradiated, photoexposed, or burned skin area can be defined as an isoradiotopic, isophototopic, or isocaumatopic response, respectively; however, the opposite may also occur when elsewhere generalized cutaneous diseases or eruptions selectively spare irradiated, photoexposed, or burned skin sites (isoradiotopic, isophototopic, and isocaumatopic nonresponse, respectively). The pathomechanisms involved in any secondary disorder occurring on irradiated or burned skin areas may be linked to locally decreased or altered lymph flow (with dysfunction of lymph drainage) on the one hand, and to fibrotic throttling or reduction of peptidergic nerve fibers (with dysfunction of neuroimmune signaling) on the other hand, resulting in a significant dysregulation of the local immune response. Future clinical observations and experimental investigations on radiation dermatitis, sunburns, and thermal or chemical skin injuries should shed new light on the mechanisms regulating regional resistance to infectious agents, local oncogenesis, and district propensity to dysimmune reactions.