 Mepolizumab is a humanised monoclonal antibody against IL5. Through a selective inhibition of eosinophilic inflammation, the agent reduces the number of eosinophils in sputum and blood, with important clinical outcomes such as a reduction of asthma exacerbations and a need for systemic glucocorticoids. The pre-licencing clinical trials have shown that selective inhibition of IL5 and presumptive removal of eosinophils reduces exacerbations by 50, but while this is a significant reduction, patients with severe asthma continue to have severe exacerbations requiring systemic corticosteroid administration. 