**8. Histopathology**

It is very characteristic and one of the diagnostic criteria of SS: a diffuse infiltrate of neutrophils located in the upper half of the dermis accompanied by intense edema. This edema causes the clinical appearance of pseudovesicular or bullous plaques. Leukocytoclasia is frequently present and may be prominent, but obvious vasculitis (neutrophils and fibrin deposits into blood vessel walls) must be absent in order to set the diagnosis. Ocasionally swollen endothelial cells, scattered eosinophils (more typical of druginduced SS), and epidermal exocytosis of neutrophils (even with formation of subcorneal pustules) can be observed. In older lesions the neutrophilic infiltrate is substituted by linfohistiocytic infiltrate (Jordaan, 1989). Requena et al (Requena et al, 2005; Requena, 2007) have described the called *histiocytoid Sweet syndrome*, characterized by a dermal infiltrate constituted by immature neutrophilic granulocytes that have an appearance indistinguishable from histiocytoid cells on optic microscopy with routine stains. The majority of this histiocytoid SS is associated with hematological malignancies, although it has recently been reported an histiocytoid SS induced by trimethoprim-sulfamethoxazole therapy with bone marrow granulocytic maturation arrest (Wu et al, 2008) and two patients with inflammatory bowel disease (Requena et al, 2005; Spencer et al, 2008). Immunohistochemical analysis is necessary when histyocites are present in SS in order to distinguish histiocytoid SS (immature neutrophils) from true histiocytes that can be present in the typical neutrophilic infiltrate, sometimes in a moderate or predominant amount (specially in older lesions) (Corazza et al, 2008).
