TNF and airway hyperresponsiveness: 2005; Wicher et al. 2017). Eosinophils exert a protective effect by day 3 due to an influx of newly divided, BrdU‐positive eosinophils that attenuate airway hyperresponsiveness (Wicher et al. 2017). Tumor necrosis factor‐alpha (TNFα) is the critical cytokine that mediates expansion of newly divided protective eosinophils in bone marrow after ozone (Wicher et al. 2017). Consequently, blocking TNFα with etanercept prevents the influx of new eosinophils into lungs and significantly worsens ozone‐induced airway hyperresponsiveness 3 days after exposure (Wicher et al. 2017).