IL-33 is becoming a central molecule in allergic asthma that addresses various cascades of innate and adaptive immune responses that lead to inflammation in the lung. Its effects are exerted via its heterodimeric receptor that consists of ST2 and the ubiquitously expressed IL-1 receptor accessory protein (ILRAcP). IL-33 integrates both innate and adaptive immunity in a unique fashion via basophils, mast cells, eosinophils, innate lymphoid cells, NK and NKT cells, nuocytes, Th2 lymphocytes and a CD34(pos) precursor cell population. These actions of IL-33 seem to be particularly strong and dominant in models with mucosal inflammation. A study in this issue of the European Journal of Immunology demonstrates that IL-33 acts, in an ST2-dependent manner, as a maturation factor for BM-derived DCs via up-regulation of CD80, CD40 and OX40L. This process is accompanied by the release of pro-inflammatory cytokines, such as IL-6, IL-1β, TNF-α and TARC/CCL17. IL-33-pre-treated DCs were significantly more potent for the generation of allergen-specific Th2-type cells with IL-5 and IL-13 production. Intratracheal administration of OVA-pulsed DCs with IL-33 significantly enhances eosinophil numbers and mucous secretion. In conclusion, IL-33 affects both the development of allergic sensitization and the development of lung inflammation in allergic asthma.