CELLS

Eosinophils represent a minor component of circulating white blood cells which can be long-lived, multifaceted granulocytes with a variety of regulatory functions.¹ They develop and differentiate during hematopoiesis in the bone marrow predominantly under the influence of IL-5 and GM-CSF, as well as IL-3, TNF-α, IFN-γ, leptin, CD40 engagement, and other factors, before migrating into the blood, after which they are terminally differentiated and do not multiply.^2,3

Eosinophil functions are diverse and include involvement in host immune response to infection, interaction and regulation of components of innate and adaptive immunity, maintenance of other immune cells, tissue remodeling, and tumor surveillance.^1,4 Through their vast cytokine arsenal and engagement of cell contact, eosinophils are capable of modulating both innate and adaptive immune responses.¹ Eosinophil granules contain four highly cationic proteins (major basic protein, eosinophil cationic protein, eosinophil peroxidase, and eosinophil-derived neurotoxins) and preformed cytokines and chemokines. Additionally, eosinophils produce leukotriene C⁴, PAFs, and prostaglandins, as well as ROS. Secretion of these eosinophil-derived mediators in response to specific stimuli, functions to modulate a wide spectrum of physiological processes including mediating cytotoxicity, chemotaxis, vascular permeability, cellular communication, and regulation and/or repair of inflammation.⁴

Elevated eosinophil levels may result from enhanced eosinophilopoiesis in the bone marrow in some clinical conditions and can cause tissue and organ damage.⁴

CD: cluster of differentiation; GM-CSF: granulocyte-macrophage colony-stimulating factor; IFN-γ: interferon gamma; IL: interleukin; PAF: platelet-activating factor; ROS: reactive oxygen species; TNF: tumor necrosis factor.