One of the five different types of white blood cell (WBC) belonging to the subgroup of WBCs called Polymorphonuclear Leukocytes . Characterised by large red (i.e. eosinophilic) cytoplasmic granules.

Eosinophil function is incompletely understood. They are prominent at sites of allergic reactions and with parasitic larvae infections ( helminths ). Eosinophil secretory products inactivate many of the chemical mediators of inflammation and destroy cancer cells. This phenomenon is most obvious with mast cell-derived mediators. Mast cells produce a chemotactic factor for eosinophils.

Produced in the bone marrow, eosinophils then migrate to tissues throughout the body. When a foreign substance enters the body, lymphocytes and neutrophils release certain substances to attract eosinophils which release toxic substances to kill the invader.
See also: Eosinophilia