Granulomatous dysimmune reactions (sarcoidosis, granuloma annulare, and others) on differently injured skin areas

Granulomatous disorders are chronic cell-mediated immune responses histologically characterized by collections of macrophages, epithelioid cells, and multinucleated giant cells. This disease spectrum often has an infectious origin, but sometimes neither an infective agent nor an inciting antigenic stimulus can be identified. The skin may be a preferential target for these disorders, especially in the areas that have been damaged by various forms of skin injury (eg, herpetic infections, trauma, thermal or solar burns, vaccinations, tattoos). These damaged skin sites frame the new concept of an immunocompromised cutaneous district (ICD), which defines a skin area with acquired immune dysregulation that can pave the way for the local onset of opportunistic disorders, such as infections, tumors, and granulomatous disorders. Sarcoidosis, granuloma annulare (GA), and forms of granulomatous vasculitis, such as Churg-Strauss syndrome (CSS) and Wegener's granulomatosis (WG), are the most common granulomatous disorders that occur in an ICD and may share common pathogenic mechanisms. Recent studies have found clinical and pathologic overlapping features across noninfectious granulomas. Although no unifying etiology exists, the development of granulomatous processes in the ICD has often been reported and the literature contains various hypotheses to explain it: (1) overactive immune response in a previously injured region with or without loss of immune tolerance; (2) overall reduced immune response; (3) retention of an exogeneous antigen or foreign body; (4) altered neural signaling; and (5) a combination of all the aforementioned processes. T helper cells, T regulatory cells, and macrophages, as well as a number of antigenic proteins, have been identified as potential contributing factors. In addition, a genetic predisposition and an intact systemic immune system are both instrumental for the persistence of local granuloma formation in the ICD.