In atopic dermatitis (AD) IgE-positive Langerhans cells (LC) may be present in the epidermis. These LC are able to capture allergens by means of their specific IgE, inducing an allergen-specific T-cell response in autologous peripheral blood T cells. Epicutaneous patch testing (EPT) may induce an eczematous reaction when IgE is present on the LC. Thus, both in vivo and in vitro, it appears that IgE may be crucial for induction of allergen-specific T-cell responses. Indeed, the cloning of infiltrating T cells from a positive 12-h EPT produced allergen-specific T cells, wheras no in vivo activated bystander T cells have yet been cloned. Moreover, greater than 85% of the T cells cloned were of Th2 phenotype after anti-CD3 and phorbol myristate acetate stimulation, whereas all clones were Th2 after allergen-specific stimulation, and they were able to induce IgE production in normal B cells. This completes the circle of events, because IgE produced by peripheral B cells may bind to LC and facilitate new allergen-specific reactions in the skin.