Eosinophils are granulocytes (white blood cells that contain granules in their cytoplasm) derived from the same progenitor cells as monocytes-macrophages, neutrophils, and basophils. They are a component of the innate immune system. Eosinophils have a variety of functions, including
Defense against parasitic infections
Defense against intracellular bacteria
Modulation of immediate hypersensitivity reactions
Eosinophils are especially important in defense against parasitic infections. However, 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.
Although they are phagocytic, eosinophils are less efficient than neutrophils in killing intracellular bacteria.
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 production and function
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.
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).
Eosinophil count
The normal peripheral blood eosinophil count varies, but it is generally accepted that a count > 500/mcL (> 0.5 × 109/L) is elevated. Peripheral eosinophilia is characterized as
Mild: 500 to 1500/mcL (0.5 to 1.5 × 109/L)
Moderate: 1500 to 5000/mcL (1.5 to 5 × 109/L)
Severe: > 5000/mcL (> 5 × 109/L)
Diurnal levels 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 the use of beta-blockers or corticosteroids, and sometimes during bacterial or viral infections.
The count can increase (eosinophilia) in allergic disorders, during certain infections (typically parasitic), and due to numerous other causes.
The circulating half-life of eosinophils is 6 to 12 hours, with most eosinophils residing in tissues (eg, the upper respiratory tract, gastrointestinal tract, skin, uterus).
The condition most commonly associated with a low eosinophil count is hypercortisolism. Data are limited but suggest an association between eosinopenia and respiratory changes during COVID-19 infection; however, pulmonary eosinophilia is not part of SARS-CoV-2 lung pathology (1).
General reference
1. Lindsley AW, Schwartz JT, Rothenberg ME: Eosinophil responses during COVID-19 infections and coronavirus vaccination. J Allergy Clin Immunol 146(1):1–7, 2020. doi: 10.1016/j.jaci.2020.04.021
