Eosinophilia is an abnormal elevation of the number of eosinophils in blood or other tissues. Eosinophilia generally indicates an inflammatory process is present.
Interleukins are involved in stimulating eosinophil production. The recently cloned IL-33 is activated in allergic conjunctivitis, and sensitizes T-lymphocytes to produce IL-4, IL-5 and IL-13 upon challenge with anti-CD3 and anti-CD28.
In blood, the normal relative percentage is 0-4%. The absolute eosinophil count, which may vary among laboratories, is between 350 and 500 cells per cubic microliter. Absolute eosinophils are measured directly if the blood cell analyzer is capable of doing so, or is calculated by multiplying the relative percentage by the total white blood cell count.
It is a characteristic of Churg-Strauss syndrome, a vasculitis with allergic components. Parasitic infections often raise the count.
Eosinophils in the nasal mucosa has been used to measure sensitivity to the house dust mite. 
- Saori Matsuba-Kitamura et al (2010), "Contribution of IL-33 to induction and augmentation of experimental allergic conjunctivitis", International Immunology 22 (6): 479-489, DOI:10.1093/intimm/dxq035
- Klaewsongkram J, Ruxrungtham K, Wannakrairot P, Ruangvejvorachai P, Phanupak P. (2003 Sep), "(Abstract) Eosinophil count in nasal mucosa is more suitable than the number of ICAM-1-positive nasal epithelial cells to evaluate the severity of house dust mite-sensitive allergic rhinitis: a clinical correlation study.", Int Arch Allergy Immunol 132 (1): 68-75.