Human eosinophils represent approximately 1% of peripheral blood leukocytes. 
However, these cells have the propensity to leave the blood stream and migrate 
into inflamed tissues. Eosinophilic inflammation is present in a significant 
proportion of patients with severe asthma. Asthma is a chronic inflammatory 
disorder that affects more than 315 million people worldwide, with 10% having 
severe uncontrolled disease. Although the majority of patients can be 
efficiently treated, severe asthmatics continue to be uncontrolled and are at 
risk of exacerbations and even death. Interleukin-5 (IL-5) plays a fundamental 
role in eosinophil differentiation, maturation, activation and inhibition of 
apoptosis. Therefore, targeting IL-5 is an appealing approach to the treatment 
of patients with severe eosinophilic asthma. Reslizumab, a humanized anti-IL-5 
monoclonal antibody, binds with high affinity to amino acids 89-92 of IL-5 that 
are critical for binding to IL-5 receptor α. Two phase III studies have 
demonstrated that reslizumab administration in adult patients with severe asthma 
and eosinophilia (≥400 cells/μL) improved lung function, asthma control, and 
symptoms. Thus, the use of blood eosinophils as a baseline biomarker could help 
to select patients with severe uncontrolled asthma who are likely to achieve 
benefits in asthma control with reslizumab. In conclusion, targeted therapy with 
reslizumab represents one step closer to precision medicine in patients with 
severe eosinophilic asthma.