Allergic asthma is becoming more prevalent—it is estimated that 15 million people in the United States alone have the condition. Animal models have provided insights into the immunologic elements that set up the condition, such as a type 2 T helper cell response and the production of interleukin-4 (IL-4) and IL-5. However, it is not clear what causes the acute reaction—airway hyperresponsiveness and severe mucus secretion, the symptoms that can lead to asphyxiation. Grünig et al. (p. 2261) and Wills-Karp et al. (p. 2258) have found that, in the mouse model of asthma, cytokine IL-13, working through activation of the α subunit of the IL-4 receptor, induces those symptoms; blocking IL-13 also blocked the symptoms. This work suggests that reagents that inhibit IL-13 action have potential therapeutic benefits (see the news story by Vogel).