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The classic presentation of ACD is a pruritic, eczematous dermatitis initially localized to the primary site of allergen exposure. S Geometric, linear, or focal patterns of involvement are suggestive of an exogenous etiology. For example, ACD from plants such as poison ivy, poison oak, or poison sumac, typically presents as a linear or streaky array of erythematous papules and vesicles. Occasionally, the sensitizing substance in these plants, an oleoresin named urushiol may be aerosolized when the plants are burned, leading to a more generalized and severe eruption on exposed areas. Transfer of the resin from sources other than directly from the plant (such as clothes, pets, or hands) may result in rashes on unexpected sites (eg, genital involvement in a patient with poison ivy). Because the mechanism of allergen exposure influences the clinical presentation, relevant historic data gathered from thoughtful questioning may prove as useful as the distribution of the lesions. Importantly, ACD will vary morphologically depending on the stage of the disease and severity of reaction. During the acute phase, lesions are marked by edema, erythema, and vesicle formation. Stronger allergens often result in vesicle formation, whereas weaker allergens often lead to papular lesion morphology, with surrounding erythema and edema. In the subacute phase, vesicular rupture leads to oozing, and scaly juicy papules associated with weeping and crusting dominate the clinical picture. Finally, the chronic phase is characterized by scaling, fissuring, and lichenification. A key symptom for ACD is pruritus, which seems to occur more typically than a symptom of a burning sensation. Moreover, there are some noneczematous clinical variants of ACD (Table 24-1) that are infrequently observed.18,19 Erythema multiforme-like ACD has been linked primarily to exotic woods, topical medicaments, and numerous miscellaneous chemicals. In contrast to true erythema multiforme, which typically presents on acral sites, lesions of erythem. multiforme-like ACD are typically at the periphery site of contact with the allergen, fever is usually absent, and mucosal involvement is rare. Purpuric ACD is mainly observed on the lower legs and/or feet and has been reported with a wide variety of allergens, including rubber and textile dyes. Lichenoid ACD is considered a rare variant that can mimic lichen planus; it is associated with color developers and metallic dyes in tattoos. Also, oral lichenoid ACD from dental amalgams can resemble typical oral lichen planus. Pigmented ACD has been mainly described in Asian-ethnicity populations. It is linked to textile dyes, cosmetics, and fragrances. Lymphomatoid ACD is based only on histopathologic criteria (presence of significant dermal infiltrate displaying features of pseudolymphoma). Nonspecific clinical signs include erythematous plaques, sometimes very infiltrated, at the site of application of the contact allergen. Allergens implicated in lymphomatoid ACD include metals (nickel and gold), para-phenylenediamine (hair dye), para-tert-butylphenol resin (glue), and dimethylfumarate (a mold inhibitor found in sachets within some furniture implicated in causing a previously severe epidemic of ACD).