Eosinophilia and the Hypereosinophilic Syndrome


The eosinophilic granulocyte is involved in the pathogenesis of diverse inflammatory processes, some of which may be beneficial to the host, such as in host defence against parasitic infection, and others are implicated in disease. In addition to being associated with allergic and atopic disease, eosinophilia can be seen in multiorgan system disorders, such as with rheumatologic and hematologic diseases, and can also be associated with single‐organ system disease involving the lungs, the heart, skin and soft tissues, and the gastrointestinal tract. Hypereosinophilia is characterised by the prolonged elevation of the peripheral blood eosinophil count, and the hypereosinophilic syndrome (HES) refers to a group of disorders marked by hypereosinophilia with multisystem target organ damage. The molecular pathogenesis of selected clonal and secondary HESs has been identified, and recognition of specific molecular defects can help to guide specific treatment approaches.

Key Concepts

  • The eosinophil is a myeloid lineage granulocyte derived from the hematopoietic stem cell.
  • Interleukin‐5 is the key cytokine involved with eosinophil lineage commitment, development, expansion, homing and activation.
  • Eosinophils produce a diverse array of mediators but there are at least four specific mediators unique to eosinophils – eosinophil cationic protein, eosinophil‐derived neurotoxin, eosinophil peroxidase and major basic protein.
  • A vast array of human diseases is associated with eosinophilia, including single‐organ system involvement and multisystem involvement.
  • The hypereosinophilic syndrome (HES) is defined by having a peripheral blood eosinophilia >1500 cell/μL for at least 1 month, with evidence of eosinophil‐dependent target organ damage, and all other aetiologies of eosinophilia excluded.
  • The myeloproliferative variant HES is characterised by abnormal PDGFRA signalling and is markedly sensitive to tyrosine kinase inhibitors targeting PDGFRA.
  • The lymphocytic variant HES is a reactive eosinophilia driven by clonal, nonmalignant T cells with abnormal immunophenotype secreting high concentrations of eosinophilpoietins such as IL‐5.

Keywords: eosinophil; eosinophilia; hypereosinophilia; hypereosinophilic syndrome; allergy; asthma; parasite


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Further Reading

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Hsieh, Fred H(Sep 2015) Eosinophilia and the Hypereosinophilic Syndrome. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002155.pub2]