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Eosinophils have myriad phlogistic activities that implicate them in disease.1–3 (See Chapter 31.) Peripheral blood eosinophilia and/or tissue infiltration by eosinophils occur in a variety of common and unusual diseases, including those of infectious, immunologic, and neoplastic etiologies. Organ-specific eosinophil disorders occur in the skin, lung, and gastrointestinal tract.4–6 Eosinophils are conspicuous in tissue sections stained with hematoxylin and eosin because of their intense avidity for eosin dye. Common dermatoses associated with eosinophils in lesional tissues include arthropod bites and drug eruptions. Parasitic infections, especially those due to ectoparasites and helminthes, typically have a marked host response with eosinophilia.7,8 Autoimmune blistering diseases, such as bullous pemphigoid and the various forms of pemphigus, often have prominent eosinophil infiltration, including histologic presentation as eosinophilic spongiosis.9,10 Infiltration of eosinophils in the subcutaneous tissues, so-called eosinophilic panniculitis, is not a specific diagnosis but rather is seen to a variable degree in diverse entities.11,12 Eosinophils may be found in Langerhans cell histiocytosis,13 cutaneous epithelial neoplasms,14 and lymphoproliferative disorders.15 Although eosinophils constitute one of the histologic features in numerous cutaneous diseases, eosinophil infiltration represents a criterion for histologic diagnosis in relatively few entities (Table 36-1). The absence, presence or number of eosinophils in skin biopsy specimens is often of limited value in reliably choosing among differential diagnoses with different and potentially important implications for clinical management, such as drug reaction versus acute graft-versus-host disease.16,17 Eosinophils play a role in certain categories of clinical reactions, particularly those characterized by edema.18 The degree of tissue eosinophil granule protein deposition in such diseases, that exhibit relatively few or no intact eosinophils, suggests that the pathogenic influence of eosinophils may be unrelated to their numbers in tissues. The degree of cutaneous eosinophil infiltration should be taken in the context of other clinical features, other histological features, and knowledge that its diagnostic power has limitations.19 However, eosinophils do have potent biological activities, particularly imparted by ...