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Nodular and diffuse dermal cellular infiltrates encompass a wide spectrum of infectious and noninfectious dermatitides as well as neoplasms. Nodular infiltrates produce discrete cellular aggregations that are separated by relatively normal dermis. Diffuse infiltrates tend to involve the entire breadth of a biopsy. They may be relegated to either the upper, middle, or lower portion of a specimen or can be confluent. Combined nodular and diffuse patterns of infiltration may also be present in some conditions. Additionally, tropism for adnexal structures, such as hair follicles and sweat ducts as well as nerve bundles, may characterize some diseases. By definition, this histopathological category exhibits absent to minimal epidermal involvement. This chapter will focus mainly on inflammatory diseases, dermatoses, and some entities in which the true nature of the infiltrate is not precisely defined (inflammatory vs neoplastic). Although many neoplasms may display these patterns (eg, melanocytic proliferations, metastatic tumors), they will be treated mainly as differential diagnoses that are discussed in depth in other sections of this book.

The conditions to be discussed are organized according to the predominant inflammatory cell type seen histopathologically and include the following seven categories: eosinophilic infiltrates, mast cell infiltrates, lymphoplasmacytic infiltrates, diffuse histiocytic infiltrates, granulomatous infiltrates (nodular and palisaded), suppurative (robust neutrophilic) and granulomatous infiltrates, and neutrophilic infiltrates. Each category thus represents a fairly extensive differential diagnostic list that is further subdivided into infectious and noninfectious conditions. Key histopathologic/clinical features that differentiate one condition from another are provided.

A low threshold for use of special stains to demonstrate microorganisms cannot be overemphasized, especially in the case of granulomatous infiltrates (whether tuberculoid, sarcoidal, palisaded, or diffuse “histiocytic”). This same rule holds true for neutrophilic and sometimes lymphocytic-plasmacellular infiltrates seen in the setting of immunodeficiency, which often have unexpected histopathologic appearances. Since cutaneous infections are considered in detail elsewhere, they are presented only in tabular form here.


Table 6-1 lists parasitic diseases that produce large numbers of tissue eosinophils.

TABLE 6-1Parasitic Causes of Diffuse Eosinophilic Infiltrates

Wells syndrome

Synonym: Eosinophilic cellulitis.

Wells syndrome (eosinophilic cellulitis) is an uncommon exaggerated dermal hypersensitivity reaction with robust tissue eosinophilia. Factors supporting this proposed mechanism include an association with an atopic diathesis, asthma, a frequent drug-based trigger, and frequent peripheral blood eosinophilia. In fact, some have proposed that Wells syndrome represents the benign end of the spectrum of the hypereosinophilic syndrome.1,2 Blood ...

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