Eosinophils have multiple relevant biological functions, including the maintenance of homeostasis, host defense against infectious agents, innate immunity activities, immune regulation through Th1/Th2 balance, anti-inflammatory, and anti-tumorigenic effects. Eosinophils also have a main role in tissue damage through eosinophil-derived cytotoxic mediators that are involved in eosinophilic
inflammation, as documented in Th2-high
asthma and other eosinophilic-associated inflammatory conditions. Recent evidence shows that these multiple and apparently conflicting functions may be attributed to the existence of different eosinophil subtypes (i.e.: tissue resident and inducible eosinophils). Therapeutic intervention with biological agents that totally deplete tissues and circulating eosinophils or, vice versa, maintain a minimal proportion of eosinophils, particularly the tissue-resident ones, could therefore have a very different impact on patients, especially when considering the administration of these
therapies for prolonged time. In addition, the characterization of the predominant pathway underlying eosinophilic
inflammation by surrogate
biomarkers (circulating eosinophils, organ-specific eosinophils levels such as eosinophil count in sputum, bronchoalveolar lavage, tissue biopsy; total circulating
IgE levels, or the use of FeNO) in the single patient with an eosinophilic-associated inflammatory condition could help in choosing the treatment. These observations are crucial in light of the increasing therapeutic armamentarium effective in modulating eosinophilic
inflammation through the inhibition in different, yet complementary ways of eosinophil pathways, such as the
interleukin-5 one (with
mepolizumab,
benralizumab,
reslizumab) or the
interleukin-4/13 one (with
dupilumab and
lebrikizumab), in severe T2-high
asthma as well as in other systemic eosinophilic associated diseases, such as
eosinophilic granulomatosis with polyangiitis and
hypereosinophilic syndrome.