Clinical Information

Eosinophils are a type of white blood cell (WBC) that derives from myeloid progenitor cells.(1) They are a critical part of the immune response to helminth and other infections and play a significant role in allergic diseases. Eosinophils are characterized by their cytoplasmic granules, which appear dark red when stained with eosin. These cytoplasmic granules contain a number of cytotoxic proteins, including major basic protein, eosinophil cationic protein, and eosinophil-derived neurotoxin (EDN). Upon activation, eosinophils degranulate, with subsequent release of these proteins into the extracellular space. These proteins exhibit a variety of activities, with EDN being a ribonuclease having antiviral activity. Eosinophils generally comprise less than 5% of the total WBC count. Eosinophilia and hypereosinophilia are defined by elevated numbers of eosinophils in the peripheral blood at 500/mcL and above and 1500/mcL and above, respectively. Apart from situations of a normal immune response, elevated eosinophil numbers can be found in allergy, asthma, malignancies, immunodeficiencies, autoimmune diseases, and eosinophilic disorders. In some cases, peripheral blood eosinophil counts may not accurately reflect elevated numbers of eosinophils found in tissues; absolute counts also do not indicate the level of eosinophil activation and degranulation. EDN concentrations have been shown to correlate with peripheral blood eosinophil counts and may provide additional information related to activation status.(2,3)