The signals that control the activation of ""Th2"", or ""allergic"", immune responses remain poorly understood. We have isolated dendritic cells, rare immune cells that initiate immune responses, from allergen-sensitised mice, and showed that when transferred into intact mice these dendritic cells have the unique ability to induce the differentiation of naïve T cells into Th2. In this application we propose to use cellular, molecular and imaging techniques to identify the cytokines and signals in the immune environment that instruct dendritic cells to initiate Th2 immune responses, and the role of mast cells in this process. In addition, we will use global gene analysis approaches to identify the dendritic cell signals that direct T cells to a Th2 differentiation pathway. Lastly, we will validate our findings using in vivo and in vitro experimental models. The resulting information will help the identification and manipulation of the environmental signals leading to Th2 activation.