Eosinophilic granulocytes, usually called eosinophils (or, less commonly, acidophils), are cells of the immune system responsible for action against multicellular parasites and certain infections in vertebrates. Along with mast cells, they also control mechanisms associated with allergy and asthma. They develop in the bone marrow (hematopoiesis) before migrating to the peripheral blood.

Such cells are eosinophilic (have acid affinity) - usually transparent, appear brick-red after staining with eosin, a red and acidic dye. The staining is concentrated in small granules in the cellular cytoplasm, which contain various chemical mediators, such as histamine and proteins such as eosinophil peroxidase, ribonuclease (RNase), deoxyribonucleases, lipase, plasminogen, and the major basic protein. These mediators are released by a process called degranulation after activation of the eosinophil, and are toxic to the tissues of the parasite and host.

In normal eosinophilic individuals constitute about 1-6% of white blood cells, and are about 12-17 micrometers in size. In normal conditions are found in the bone marrow, at the junction between the cortex and thymus medulla, gastrointestinal tract, ovary, uterus, spleen and lymph nodes, but not in organs such as lung, skin and esophagus. The presence of eosinophils in the latter of these organs is associated with some diseases. Eosinophils persist in circulation for 8-12 hours, and can survive in tissues for an additional 8-12 days in the absence of stimulation.