Eosinophils are granulocytes and generally make up about 1-5 % of the circulating leukocytes. They are slightly larger than neutrophils, usually 12 to 16 μm in diameter. The nucleus occupies a rela tively small part of the cell. The cytoplasm is usually colorless, but it may be faintly basophilic. It is crowded with spherical acidophilic granules, which stain red-orange with eosin and are larger and more distinct than neutrophilic granules.
The function of eosinophils is phagocytosis of antigen-antibody complexes.They become active in the later stages of inflammation. Eosinophils respond to allergic and parasitic diseases. Eosinophils play an important role in the defense against parasitic infections. They have granules that contain histamine used to kill foreign cells in the body and proteolytic enzymes that damage parasitic worms. Eosinophils also phagocytize cell debris, but to a lesser degree than neutrophils, and do so in the later stages of inflammation.
Eosinophilia, an increase in the number of eosinophils above normal, is associated with a wide variety of conditions, but especially with allergic reactions, drug reactions, certain skin disorders, parasitic infestations, collagen vascular diseases, Hodgkin’s disease, and myeloproliferative diseases. Eosinopenia, or decreased number of eosinophils, is seen with hyperadrenalism.