What Are Eosinophils?

Eosinophils, also written eosinophiles, are granulocytic white blood cells characterized by a bilobed nucleus and by cytoplasmic granules that stain intensely with the acidic dye eosin. They make up roughly one to four percent of circulating leukocytes in healthy adults, a small share that understates their importance, since the majority of the body's eosinophils reside in tissue rather than in blood, particularly in the gastrointestinal tract, the thymus, and the bone marrow. Their name comes from the staining property that Paul Ehrlich used to distinguish them from neutrophils and basophils in the 1870s.

Eosinophils develop in the bone marrow from a common myeloid progenitor shared with neutrophils, basophils, and monocytes. Differentiation is driven principally by interleukin-5, with supporting contributions from interleukin-3, granulocyte-macrophage colony-stimulating factor, and interleukin-33. Mature cells circulate for a matter of hours before migrating into tissue under the influence of the eotaxin family of chemokines acting on the CCR3 receptor, where they may survive for days to weeks.

Granule Contents and Effector Mechanisms

The defining feature of the eosinophil is its secondary granule, a crystalloid structure with a core of major basic protein surrounded by a matrix containing eosinophil cationic protein, eosinophil-derived neurotoxin, and eosinophil peroxidase. These are strongly cationic and cytotoxic, capable of disrupting the membranes of helminth larvae and of host tissue alike. Release occurs by classical exocytosis, by piecemeal degranulation in which individual mediators are shuttled out in vesicles, and by cytolysis that leaves intact free granules in tissue. Eosinophils also generate reactive oxygen species, synthesize leukotrienes and prostaglandins, and can extrude extracellular DNA traps that immobilize bacteria, a set of mechanisms reviewed in work on the pleiotropic functions of eosinophils.

Immune Regulation and Homeostasis

The older view of the eosinophil as a single-purpose antiparasitic effector has been substantially revised. Eosinophils present antigen, secrete a wide panel of cytokines including interleukin-4 and interleukin-13, and interact with mast cells, T cells, and innate lymphoid cells to shape type 2 immune responses. In the healthy gut and adipose tissue they support metabolic and mucosal homeostasis, and in the thymus they participate in the selection of developing lymphocytes, roles set out in a review of eosinophils and eosinophilic immune dysfunction in health and disease. Reported interaction with nerve endings in the airway and gut has opened a further line of research on neuroimmune signaling.

Eosinophilia and Clinical Measurement

An absolute eosinophil count above about 500 cells per microliter defines eosinophilia, and counts above 1,500 sustained over time define hypereosinophilia. The common causes are allergic disease, helminth infection, drug reactions, certain malignancies, and adrenal insufficiency, while clonal eosinophilia arises from myeloid neoplasms carrying rearrangements of PDGFRA, PDGFRB, or FGFR1. Counting is performed by automated hematology analyzers using flow cytometric light scatter and peroxidase or fluorescence channels, with manual differential review reserved for flagged samples. Eosinophils also feature in autoimmune conditions, both as participants in tissue damage and as regulatory cells, a dual role examined in reviews of eosinophils in autoimmune disease. Blood and sputum eosinophil counts now guide the use of monoclonal antibodies targeting interleukin-5 and its receptor in severe asthma.

Applications

The study and measurement of eosinophils have relevance in a wide range of fields, including:

• Clinical hematology, through automated differential white cell counting
• Respiratory medicine, where eosinophil counts stratify asthma phenotypes and biologic therapy
• Allergy and immunology diagnostics
• Parasitology and tropical medicine surveillance
• Gastroenterology, in the diagnosis of eosinophilic esophagitis and related disorders
• Flow cytometry and image-based cell classification, including machine learning approaches to blood smear analysis
• Drug development targeting interleukin-5 signaling and eosinophil trafficking