S1C30P20-1a & b: The eosinophil is a specialized cell with distinctive, acidophilic, cytoplasmic granules. The granules are larger and more acidophilic than those of a neutrophil; the nucleus of an eosinophil is usually bi-lobed. This special cell is a store of catalytic enzymes and toxic proteins. It may secrete the stored products or, by disintegrating, release its granules into the interstitium. It is a migratory cell with a special affinity for allergic reactions. It is often emphasized as a significant reacting cell in helminth infestations, and is a common component of the infiltrates associated with arthopod bites, scabies, and eosinophilic cellulitis (Well’s syndrome). It is variably represented in the infiltrates of many drug eruptions, including interstitial disorders such as eosinophilic fasciitis (Shuman’s syndrome). It is also found in variable numbers in the infiltrates of nummular eczema. It may be found in both dermal and epidermal infiltrates in disorders such as bullous pemphigoid, or pemphigus. In epidermal infiltrates, eosinophils often are associated with intercellular edema and intraepidermal vesicles (eosinophilic spongiosis). Edema and eosinophilic infiltrates go hand in hand;  in a temporal sequence, histiocytes commonly follow eosinophils. If not for the contributions of “molecular pathologists,” it might be tempting to attribute the relationship between eosinophil-rich infiltrates and aggregated histiocytes to the insolubility of the products of eosinophils. In the above illustrations, the patterns in the epidermis are psoriasiform and spongiotic vesicular. The papillary dermis is widened and edematous. The presence of eosinophils in the infiltrates qualifies the reaction as eosinophilic spongiosis. This is a generic category which includes, among other options, harbingers of either bullous pemphigoid or pemphigus. Other disorders may occasionally present as an eosinophilic spongiosis only to be revealed as a more specific disorder at a later date. 

S1C30P20-2: In this field of a lesion of eosinophilic spongiosis, the epidermal pattern is somewhat reticulated. Blue arrows point to eosinophils and red arrows to neutrophils.

S1C30P20-3a, b&c (angiolymphoid hyperplasia with eosinophilia): A vascular plexus is important for the delivery of inflammatory cells to appropriate sites. A particular vessel (the post-capillary venule with high endothelial cells) is specially adapted to allow the egress and ingress, of inflammatory cells, particularly lymphocytes and histiocytes. Those that leave the vessels beyond the domain of lymph nodes are available for interactions in immune processes. Those that enter the circulation may seek out lymph nodes, and egress from the vessels into the domain of lymphoid tissue. In lymph nodes, the newly arrived lymphocytes and histiocytes may provide antigenic and genetic information that will be important for the recruitment of other  cells, such as immunogenically dedicated lymphoid cells. Angiolymphoid hyperplasia with eosinophilia is a tumor characterized by a proliferation of distinctive vessels and by the sequestration of lymphocytes, histiocytes, eosinophils, and plasma cells in the vicinity of the abnormal vessels. The vessels are lined by epithelioid, or “histiocytoid,” endothelial cells in the areas showing diagnostic changes. In some areas, particularly at the periphery of variants which are circumscribed and subcutaneous, the vessels have the qualities of post-capillary venules. Vessels with the qualities of post-capillary venules are outlined by green arrows in S1C30P20-3c. 

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