Eosinophils are white blood cells comprising only 1–2% of blood leukocytes, but found in greater concentrations residing within the gastrointestinal tract, thymus, uterus as well as adipose and lymphoid tissues of healthy individuals. The number of eosinophils increases in association with inflammatory diseases including bronchial asthma, helminth infections, hyper‐eosinophilic syndromes and allergic diseases. Eosinophils exhibit cytotoxic effects on microbes and helminths and damage body tissues through secretion of their granule‐derived cationic proteins, and have therefore earned a reputation as end‐stage effector cells. However, the paradigm of eosinophils as strictly end‐stage effectors is evolving. It is now recognised that eosinophils also store a vast array of pre‐formed cytokines within intracellular granules, and undergo distinct modes of degranulation. In large part due to their secretory products, eosinophils participate in health and disease, from metabolic homeostasis to adaptive immunity.

Key Concepts

  • Eosinophils are present only transiently in the peripheral blood and are primarily tissue‐dwelling cells within the gastrointestinal tract, thymus, uterus as well as lymphoid and adipose tissues.
  • The number of eosinophils increases during infection with parasitic helminths and allergic inflammatory diseases, such as bronchial asthma.
  • Eosinophil intracellular granules react avidly with eosin dye because of their storage of strongly cationic proteins, which exert cytotoxic effects on parasites, microbes, tissues and cells.
  • Because of their secretion of cytotoxic granule‐derived cationic proteins, eosinophils are widely recognised as end‐stage effector cells; however, eosinophils also store and secrete a vast array of cytokines and participate in diverse aspects of health and disease, from metabolic homeostasis to adaptive immunity.
  • Eosinophils release their granule contents into the extracellular space through three distinct modes of degranulation: classic exocytosis, piecemeal degranulation and cytolysis.
  • Recent data suggest that cell‐free, extracellularly deposited eosinophil granules expulsed during eosinophil cytolysis remain secretory‐competent organelles within tissues.

Keywords: asthma; hypersensitivity diseases; allergic inflammation; helminth; interleukin‐5; cationic protein; piecemeal degranulation; cytolysis; cytokine

Figure 1. Eosinophil morphology. At the upper left is a characteristic eosinophil with coarse granules and a bi‐lobed nucleus. At the lower right is neutrophil with its typical multi‐lobed nucleus and faintly staining granules. The smaller pale reddish‐staining cells in the background are erythrocytes; these cells are biconcave, hence doughnut shaped, and have an apparent hole in the centre.
Figure 2. Transmission electron microscopy of a human eosinophil. This cell is characterised by a major population of specific granules (Gr) with a unique morphology – an internal, often electron‐dense, crystalline core and an outer electron‐lucent matrix surrounded by a delimiting trilaminar membrane. Note the typical bi‐lobed nucleus (Nu), lipid body (LB) and large tubular carriers (arrowheads). The inset shows secretory granules and a tubular vesicle at higher magnification. Bars, 500 nm; 300 nm (inset). Source: Reproduced from Spencer et al. () © US National Library of Medicine, National Institutes of Health.
Figure 3. Charcot–Leyden crystals. The Charcot–Leyden crystals have a characteristic morphology consisting of hexagonal dipyramids. These crystals are formed from eosinophils and also basophils; the crystals above were derived from purified eosinophils. The frame (a) (160×) shows the purity of this preparation, and the frame (b) (400×) shows the distinctive appearance of the crystals. Source: Reproduced from Gleich et al. () © US National Library of Medicine, National Institutes of Health.
Figure 4. Biological effects of eosinophil‐derived cytokines in health and disease. Eosinophils secrete numerous cytokines with varied biological functions in health (left panels) and disease (right panels). Shown is an abridged list of eosinophil‐derived cytokines. M2, alternatively activated macrophage; PC, plasma cell; B, B cell; BV, blood vessel; Eos, eosinophil; T, T cell; DC, dendritic cell. Source: Reproduced with permission from Melo et al. () © John Wiley and Sons.


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Spencer, Lisa A(Feb 2016) Eosinophils. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001126.pub2]