An eosinophil is a leukocyte (i.e., white blood cell) that, in the mature form has a nucleus that usually has two lobes connected by a slender thread of chromatin, and cytoplasm containing coarse, round granules that are uniform in size and stainable by eosin. They normally make up 1 to 4 percent of all white cells, and a normal total eosinophil count is less than 350 cells/mcL (cells per microliter).
Eosinophils attack foreign substances too large for other cells to surround using the mechanism of phagocytosis. They produce leukotriene C4 and platelet activation factor (PAF).  PAF causes platelets to aggregate (i.e., clump). Especially infectious parasites produce inhibitors of platelet aggregation.
In the creation of white cells, the original progenitor is the pluripotent stem cell. Under the influence of interleukins IL-5, they form committed stem cells (i.e., progenitors). The cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) causes differentiation into eosinophils. Like neutrophils, the juvenile cells begin with a single smooth nucleus, but form lobes as they mature.
Role in specific disorders
During exacerbations of asthma, eosinophils migrate to the bronchial mucosa; the blood total eosinophil count and the number of hypodense eosinophils correlate with the severity of the attack. One of the most important inflammatory mediators they release is leukotriene C4, which is a potent vasoconstrictor. 
Churg-Strauss syndrome, also known as allergic granulomatous angiitis, is a granulomatous small-vessel vasculitis. An elevation in eosinophils is one of the criteria for its diagnosis. The American College of Rheumatology (ACR) has proposed 6 criteria for the diagnosis of Churg-Strauss syndrome. The presence of 4 or more criteria yields a sensitivity of 85% and a specificity of 99.7%. These criteria are:
- asthma (wheezing, expiratory rhonchi)
- eosinophilia of more than 10% in peripheral blood
- paranasal sinusitis
- pulmonary infiltrates (may be transient)
- histological proof of vasculitis with extravascular eosinophils
- mononeuritis multiplex or polyneuropathy
- National Library of Medicine, Medical Subject Headings (MeSH)
- "Complete Blood Count", MedLine Plus
- "Eosinophil count, absolute", MedLine Plus
- Ganong, William F. (Nineteenth edition, 1999), Review of Medical Physiology, Appleton & Lange,pp. 494-495
- Ganong, p. 508
- Liu LX, Weller PF (1992 April), "Intravascular filarial parasites inhibit platelet aggregation. Role of parasite-derived prostanoids.", Clin Invest 89 (4): 1113–1120., DOI:10.1172/JCI115691.
- Laviolette, M et al. (1995), "Blood eosinophil leukotriene C4 production in asthma of different severities", Eur Respir J 8: 1465-1472
- Farid-Moayer, Mehran & Sandra L Sessoms (Jan 5, 2007), "Churg-Strauss Syndrome", eMedicine
- Masi AT, Hunder GG, Lie JT, Michel BA, Bloch DA, Arend WP, et al. (1990), "The American College of Rheumatology 1990 criteria for the classification of Churg-Strauss syndrome (allergic granulomatosis and angiitis).", Arthritis Rheum 33: 1094-1100