eosinophil

An eosinophil is a type of white blood cell belonging to the granulocyte family. It accounts for about 1–6% of circulating leukocytes in humans. Eosinophils have a bilobed nucleus and cytoplasmic granules that stain with eosin, giving the cell its name. The granules contain several toxic proteins, including major basic protein, eosinophil cationic protein, eosinophil-derived neurotoxin, and eosinophil peroxidase.

Eosinophils develop in the bone marrow from myeloid progenitors under signals such as interleukin-5 (IL-5), IL-3, and GM-CSF. After release into the bloodstream, they circulate for roughly 8–18 hours, with longer residence in tissues, especially during inflammation. They migrate to sites of inflammation in response to chemokines such as eotaxins (eotaxin-1/CCL11) and other mediators.

Functions of eosinophils include defense against parasitic helminths through the release of cytotoxic granule proteins and reactive oxygen species. They also participate in shaping immune responses by releasing cytokines and lipid mediators. In allergic diseases and asthma, eosinophils can contribute to tissue damage and airway hyperreactivity through degranulation and inflammatory mediator release.

Clinical significance is noted with eosinophilia, an elevated eosinophil count that can arise from parasitic infections, allergic diseases, drug reactions, certain autoimmune conditions, and some malignancies such as Hodgkin lymphoma. Diagnosis relies on complete blood counts and differential. Treatments target underlying causes; corticosteroids reduce eosinophil numbers, and biologics against IL-5 or the IL-5 receptor (for example, mepolizumab, reslizumab, benralizumab) are used for eosinophilic asthma and related disorders.