Eosinophils are white blood cells (WBCs) of the granulocytic lineage, which also includes neutrophils and basophils [1]. They are likely involved in host immune response to infection, tissue remodeling, tumor surveillance, and maintenance of other immune cells [2].
Eosinophils develop and differentiate in the bone marrow under the influence of interleukin (IL)-5, IL-3, and granulocyte-macrophage colony-stimulating factor (GM-CSF) [3].
Normal levels — In peripheral blood, an absolute eosinophil count (determined by multiplying the total WBC count by the percentage of eosinophils) of 0 to 500/microL (<0.5 x 109/L) is typically considered normal. Defining the normal range by percentage of WBC can be problematic because differences in absolute neutrophil counts between ethnic groups can dramatically impact the percent of eosinophilia [4]. Blood eosinophil counts have been reported to vary within the same person at different times of day and on different days, both in individuals with eosinophilic disorders and in healthy volunteers [5-7]. However, results are inconsistent among studies, and the variability in counts is rarely large enough to impact care.(??)
Eosinophils are primarily tissue-dwelling cells; they are several hundred-fold more abundant in tissues than in blood [8]. In healthy individuals, eosinophils can be found in the digestive tract (although not the esophagus), spleen, lymph nodes, thymus, mammary glands, and uterus [2]. Recruitment of eosinophils to these tissues is mediated mainly by eotaxins, a family of chemokines (ie, cytokines that stimulate cell migration) that bind to the eosinophil chemokine receptor CCR3.