Many dermatological diseases may be considered to be disorders of altered reactivity, in which the skin responds abnormally to a variety of stimuli, involving either immune or non-immune mechanisms. For example, altered immunological mechanisms are involved in the pathogenesis of allergic contact dermatitis, atopic dermatitis (Chapter 2), and vasculitis (Chapter 7), whereas dermographism is an example of an altered response to a physical stimulus. Included in this section are the urticarias, drug

reactions, the photodermatoses, graft-versus-host disease, and less well-defined disorders such as annular erythema, Sweet’s syndrome, eosinophilic cellulitis (Wells’ syndrome), and Behçet’s disease. The overlap of these disorders with internal medicine is readily apparent.