Eosinophils are effector cells of the innate immune system that play a key role in the body's defense against parasites and tumors. Their main function is to eliminate biological targets, such as helminths, protozoa, bacteria, and tumor cells. The pathological circumstances that trigger an increase in the number of eosinophils in tissues or peripheral blood are very varied. The literature includes very few review articles dealing specifically with eosinophilic dermatoses, and attempts to draw up a preliminary classification are even rarer.1

In 2001, Byrd et al.2 coined the term eosinophilic dermatosis of myeloproliferative disease, for which they proposed the following clinical and pathological diagnostic criteria: a pruritic papular or vesiculobullous eruption refractory to standard treatment; a concurrent diagnosis of blood dyscrasia or hematologic malignancy; an eosinophil-rich lymphohistiocytic infiltrate in the superficial and deep dermis; and the exclusion of other causes of tissue eosinophilia. This description corresponds to the dermatosis originally described in 1965 by Weed3 as a mosquito bite-like reaction. More recently, a new nomenclature has been proposed: eosinophilic dermatosis of hematologic malignancy.4 It is a reactive process without specific hematologic cell infiltration. The etiology and pathogenesis is poorly understood, but it is thought to be associated with the immune dysregulation found in conditions such as chronic lymphocytic leukemia.

Our journal has already published a small case series of this skin condition.5 In this issue of Actas Dermosifiliográficas, Lucas-Truyols et al.6 review 4 new typical examples and, for the first time, 1 case associated with mycosis fungoides. The great interest of this condition, and all skin disorders linked to systemic diseases, is that its prompt recognition may have diagnostic and, in some cases, prognostic value with regard to the underlying hematologic disease.