IL-5 is an inflammatory cytokine that is critical for the development, maturation, and survival of eosinophils. In 1999, two anti-IL-5 hMabs were described—mepolizumab and reslizumab, which bind to the IL-5 protein and prevent subsequent binding to the IL-5 receptor. Initial studies of both hMabs in severe refractory asthma were disappointing., This is because they were initially evaluated in unselected patients with severe refractory asthma, many of whom did not have persistent airway eosinophilia. As a result of the identification of patient populations with severe refractory eosinophilic asthma, more focused studies were initiated. One of these studies examined the benefit of mepolizumab in patients with prednisone-dependent severe eosinophilic asthma. This study showed that treatment with mepolizumab for 6 months allowed reduction (and discontinuation in most patients) of prednisone without loss of asthma control or asthma exacerbation. A second study confirmed a benefit in patients with severe eosinophilic asthma, with a 50% reduction of severe asthma exacerbations over 1 year of treatment. Mepolizumab subsequently went into clinical trials with much larger populations of patients with severe eosinophilic asthma; it was confirmed that it produced a benefit for reducing severe asthma exacerbations (Fig 1), reduced oral steroid requirements, and slightly improved FEV1 values (Fig 1)., Similarly, with the second anti-IL-5 hMab, reslizumab, studies in patients with severe refractory asthma showed a significant reduction in asthma exacerbations and improvement in lung function.