Gilda Varricchi1,2, 
Gianenrico Senna3, 
Stefania Loffredo1,2, Diego Bagnasco4, 
Matteo Ferrando4 and 
Giorgio Walter Canonica5* 1Division of Clinical Immunology and Allergy, Department of Translational Medical Sciences, School of Medicine, University of Naples Federico II, Naples, Italy
2Center for Basic and Clinical Immunology Research (CISI), University of Naples Federico II, Naples, Italy
3Asthma Center and Allergy Unit, Verona University, General Hospital, Verona, Italy
4Allergy and Respiratory Diseases, DIMI Department of Internal Medicine, IRCCS AOU San Martino-IST, University of Genova, Genova, Italy
5Personalized Medicine Clinic Asthma and Allergy Humanitas Clinical and Research Center, Department of Biomedical Science, Humanitas University, Rozzano, Milano, Italy
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.