Allergen challenge, eosinophils and the long road to asthma endotypes

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Inhaled allergen challenges (IAC) have been utilised as tools to understand the clinical, physiological and pathobiological responses to allergens in asthmatic and atopic individuals for close to 75 years [1, 2]. Sensitised individuals respond with an early/immediate fall in forced expiratory volume in 1 s (FEV₁): the early asthmatic response (EAR). Following a return to baseline (within 60 min), 30–50% of individuals also experience a later fall in FEV₁, beginning around 4–6 h post-inhalation which lasts hours to even days: the late asthmatic response (LAR). For at least 50 years, allergen challenges have been used as early screens for the efficacy of novel therapeutics, with suggestions that inhibition of allergic responses is an overall (and necessary) predictor of clinical success of the drug [2].