Eosinophils are granulocytes derived from the same progenitor cells as monocytes-macrophages, neutrophils, and basophils. The precise functions of eosinophils are unknown. Although they are phagocytic, eosinophils are less efficient than neutrophils in killing intracellular bacteria. And although eosinophilia commonly accompanies helminthic infections and eosinophils are toxic to helminths in vitro, there is no direct evidence that they kill parasites in vivo. Eosinophils may modulate immediate hypersensitivity reactions by degrading or inactivating mediators released by mast cells, such as histamine, leukotrienes (which may cause vasoconstriction and bronchoconstriction), lysophospholipids, and heparin. Prolonged eosinophilia may result in tissue damage by mechanisms that are not fully understood.

Eosinophil granules contain major basic protein and eosinophil cationic protein; these proteins are toxic to several parasites and to mammalian cells. These proteins bind heparin and neutralize its anticoagulant activity. Eosinophil-derived neurotoxin can severely damage myelinated neurons. Eosinophil peroxidase, which differs significantly from peroxidase of other granulocytes, generates oxidizing radicals in the presence of hydrogen peroxide and a halide. Charcot-Leyden crystals are primarily composed of phospholipase B and are located in sputum, tissues, and stool in disorders in which there is eosinophilia (eg, asthma, eosinophilic pneumonia).

The normal peripheral blood eosinophil count is < 350/μL, with diurnal levels that vary inversely with plasma cortisol levels; the peak occurs at night and the trough in the morning. The eosinophil count can decrease with stress, with use of β-blockers or corticosteroids, and sometimes with bacterial or viral infections. The count can increase with allergic disorders, with certain infections (typically parasitic), and from numerous other causes (see Eosinophilic Disorders: Eosinophilia). The circulating half-life of eosinophils is 6 to 12 h, with most eosinophils residing in tissues (eg, the upper respiratory and GI tracts, skin, uterus).

Eosinophil production appears to be regulated by T cells through the secretion of the hematopoietic growth factors granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and interleukin-5 (IL-5). Although GM-CSF and IL-3 also increase the production of other myeloid cells, IL-5 increases eosinophil production exclusively.

Last full review/revision November 2009 by Mary Lynn R. Nierodzik, MD

Content last modified February 2012