Eosinophils (or less commonly acidophiles) are myeloid granulocytes and form one of the main types of white blood cells. Their counts are routinely measured as part of a full blood count. They have important roles in fighting parasitic infections, but it is increasingly recognised that they have a vital role in the immune defence against bacteria and viruses. 

  • normal level: <450 cells/μl
  • eosinophilia: >450-500 cells/μl
  • hypereosinophilia: >1500 cells/μl

Eosinophils develop in the bone marrow from myeloid precursor cells under stimulation from interleukins, namely IL-3, IL-5 and GM-CSF, and maturation takes ~1 week 1.

Eosinophils have a relatively short half-life within the circulation and attach themselves to the endothelial cells of the vascular wall to enter the tissues via diapedesis. The release of proteins, cytokines and chemokines help produce the acute inflammatory response while also directly killing invading organisms.2

They are particularly abundant in the mucosa of the GI, respiratory and urinary tracts where they are the primary defence against parasitic organisms. Their numbers are also increased in allergic responses such as asthma.  

Eosinophil derives from the Ancient Greek root "phil" meaning love. Eosin is a histological acidic dye, therefore eosinophil means "eosin loving".

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Article information

rID: 60124
Section: Pathology
Synonyms or Alternate Spellings:
  • Eosinophils
  • Acidophil

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