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Notes: Atopic dermatitis (AD) is considered a T-cell mediated disease. Activated T-cells, mainly of the CD4-subtype. are abundantly present in lesional AD skin. Although not many intact eosinophils are present, deposits of eosinophil derived major-basic-protein (MBP) and eosinophil-cationic-protein (ECP) suggest eosinophil involvement. After patch testing AD patients with aeroallergens, an eczematous reaction develops after 24 48 hr at the site of application. This patch test reaction shows macroscopic resemblance to lesional AD skin and does not take place in normal individuals, asthma and allergic rhinitis patients. Lymphocytes together with eosinophils infiltrate into the dennis 26 hr after allergen application. Twenty-four to forty-eight hours after patch testing, eosinophils are in an activated state since they release ECP (being EG2-positive). At this point in time eosinophils have also infiltrated the epidermis. Here they are EG2-negative. Forty-eight to seventy-two hours after patch testing the eczematous reaction decreases. This coincides with disappearance of eosinophils from both the dermis and the epidermis; then, a dendritic staining pattern can be observed in the epidermis with anti-eosinophil peroxidase. Thus, eosinophils infiltrate the dermis and epidermis after patch testing AD patients with aeroallergens and release part of their granular constituents. Recent in vitro investigations revealed that eosinophils from the circulation of AD patients react more powerfully in in vitro test systems such as chemi luminescence, chemotaxis and endolhelial adherence and transmigration. 11 is very likely that this activated (= primed) state is caused by the influence of lymphocyte-derived cytokines like IL-3, IL-5 and GM-CSF, since activated lymphocytes in the circulation (and tissue) may release these cytokines. The primed state of the eosinophils may facilitate tissue inlillration. The subsequent activation of eosinophils within the tissue leading to mediator release and the function of these mediators need to be further elucidated. The close similarity between the cellular events after a patch test reaction to aeroallergens in AD patients and those present in lesional AD skin suggests that the patch test reaction may be a helpful in rim model to study the pathogenesis of AD. The prominent involvement of lymphocytes and eosinophils in this reaction also suggests some similarity with late phase reactions (LPR) observed in the skin after intracutaneous allergen challenge.