Pyoderma gangrenosum is a rare, ulcerative, cutaneous condition. First described in 1930, the pathogenesis of pyoderma gangrenosum remains unknown, but it is probably related to a hyperergic reaction. There are various clinical and histological variants of this disorder. Pyoderma gangrenosum often occurs in association with a systemic disease such as inflammatory bowel disease, rheumatologic disease, paraproteinaemia, or haematological malignancy. The diagnosis, mainly based on the clinical presentation and course, is confirmed through a process of elimination of other causes of cutaneous ulcers. Local treatment may be sufficient for mild disease, while for severe cases, systemic immunosuppressants are the mainstay. Long-term treatment with these agents is often required, but this can expose patients to adverse side-effects.

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Systemic immunodeficiency is known to facilitate the onset of opportunistic infections, tumours and immune disorders in any district of the body. There are clinical events, such as chronic lymphoedema, herpetic infections, vaccinations and heterogeneous physical injuries which can selectively damage and immunologically mark the cutaneous district they act upon. After the causing event has disappeared, the affected district may appear clinically normal, but its immune behaviour is often compromised forever. An immunocompromised district becomes a site which is particularly susceptible to subsequent outbreaks of opportunistic infections, tumours and immune disorders confined to the district itself. In this review, there is an ample case-report collection of opportunistic disorders (infectious, neoplastic, immune) which appeared in immunocompromised districts. The cases have been grouped according to the clinical settings responsible for the local immune imbalance: regional chronic lymphoedema; herpes-infected sites, which feature the well-known Wolf's isotopic response; and otherwise damaged areas, comprising sites of vaccination, ionizing or UV radiation, thermal burns and traumas. Whatever the immunocompromising factor, a common denominator which facilitates the occurrence of tumours, infections and dysimmune reactions in an immunocompromised district may reside in locally hampered lymph drainage and/or locally altered neuromediator signalling. In fact, any obstacle to the normal trafficking of immunocompetent cells through lymphatic channels or any interference with the signals that the neuropeptides and neurotransmitters released by peripheral nerves send to cell membrane receptors of immunocompetent cells, can significantly alter the local immune response, thus paving the way for heterogeneous opportunistic disorders in the immunocompromised district.