In contrast to Th1 immune responses against pathogenic viruses and bacteria, the incipient events that generate Th2 responses remain less understood. Part of the difficulty in identifying universal operating principles stems from the diversity of entities against which cellular and molecular Th2 responses are produced. Indeed, such responses are launched towards harmful macroscopic parasites and noxious substances such as venoms but also against largely innocuous allergens. This suggests that the canonical understanding about sensing and recognition applied to Th1 responses may not be translatable to Th2 responses. This review will discuss processes and signals known to occur in Th2 responses, particularly in the context of food allergy. We propose that perturbations of homeostasis at barrier sites induced by external or internal subverters that either activate the immune system or lower its threshold activation are the major requirement for allergic sensitization. Innate signals produced in the tissue under these conditions equip dendritic cells with a program that shapes an adaptive Th2 response.