Allergic inflammation plays crucial roles in allergic diseases such as asthma. These diseases impact the lives of millions, and it is thus important to understand why and how allergic responses start. Corinne Cayrol and Anais Duval in the team of Jean-Philippe Girard at IPBS have discovered a general mechanism for the induction of allergic inflammation that is based on detection of biochemical activities associated with environmental allergens such as molds, house dust mites and pollens. The protein involved in this sensing mechanism, interleukin-33, belongs to a family of soluble messengers that allow cells to communicate. When it encounters allergens, this sensor is fragmented into small parts that trigger the activation of white blood cells implicated in allergic inflammation. Interfering with this mechanism could represent a new approach for reducing allergic responses in asthma and other allergic diseases. This discovery establishes a direct link between genetics and environment in asthma, since the gene encoding interleukin-33 is one of the major asthma susceptibility loci.