Eosinophils: Friends or Foes?

The roles eosinophils are recognized to play in health and disease continue to evolve. Formerly, eosinophils were believed to fill a primary role in host defense against helminths, an intermediary one in the propagation of allergic conditions, and a pathologic one in clinical conditions characterized by systemic eosinophilia and eosinophilic infiltration of target organs. Eosinophils are increasingly understood to be positioned centrally within mammalian immune and inflammatory networks, possessing receptors for an array of inflammatory mediators and capable of producing numerous proinflammatory and homeostatic mediators. The concept has emerged that eosinophils play a major role in the modulation of allergic inflammation and in the repair of damaged tissues in diseases characterized by eosinophilic infiltration. Possible new emerging roles for eosinophils include neoplasm surveillance, tissue remodeling during puberty and pregnancy, and the restructuring of adipose tissue. The eosinophil granulocyte line appeared hundreds of millions of years ago during the evolutionary process and continues to be retained by all vertebrate species. This is strong evidence that although all the beneficial roles of eosinophils have yet to emerge, eosinophils, on balance, must be considered friends and not foes. (C) 2018 American Academy of Allergy, Asthma & Immunology