Eosinophils in skin disease: bystanders or pathogenic players?

Eosinophils, i.e. eosinophilic granulocytes, play a crucial role in the physiologic host defense against parasites but are also promoters of allergic tissue inflammation of the Th2 type. They are central players in the so-called eosinophilic dermatoses, such as Wells syndrome, hypereosinophilic syndrome, eosinophilic granulomatosis with polyangiitis, eosinophilic fasciitis, and eosinophilic folliculitis among others. Eosinophils originate from the bone marrow, enter the skin under the influence of chemokines and other chemotactic factors, and are linked to a Th2 type cytokine milieu of IL-3, IL-4, IL-5, IL-13 and IL-31. We herewith describe the cardinal features as well as pathophysiological aspects of eosinophilic dermatoses. Moreover, we outline novel targeted treatments, including antibodies against key cytokines governing eosinophil differentiation, migration and function.