Several lines of evidence suggest that deficiency of eosinophils is not 
associated with any characteristic abnormality. Patients lacking eosinophils, in 
the setting of immunodeficiency or as a consequence of IgG-mediated eosinophil 
precursor destruction, do not display any distinguishing abnormalities related 
to eosinophil reduction. The observation that eosinophil-deficient mice do not 
display any distinctive syndrome or failure of their health is evidence that, 
under ordinary laboratory conditions, the eosinophil does not play a critical 
role in the well-being of mammals. Observations that monoclonal antibodies to 
interleukin-5 (IL-5) are well tolerated appear unsurprising in light of these 
findings. For example, patients with the hypereosinophilic syndrome have 
received mepolizumab, an anti-IL-5 monoclonal antibody, for as long as 6 years 
and have not developed any characteristic set of adverse events. Safety data for 
reslizumab, another anti-IL-5 monoclonal antibody, and benralizumab, a 
monoclonal antibody to the IL-5 receptor α-chain, are comparatively limited, 
especially for benralizumab, although reports of administration of these 
antibodies to humans suggest that they are well tolerated. Thus, data to the 
present suggest that reduction of eosinophils appears to have no characteristic 
ill effects on normal health, and monoclonal antibodies that deplete eosinophils 
have the potential to be widely employed in the treatment of 
eosinophil-associated diseases.