Last revised by Daniel J Bell on 15 Nov 2021

Eosinophils, also less commonly known as acidophils, 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 are increasingly recognized as having a vital role in the immune defense against bacteria and viruses. 

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 defense 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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