Immune Functions of Human Eosinophils
For more than 100 years, eosinophils have been readily recognized by microscopic examination of blood, bone marrow, and other tissues due to their property of staining strongly with eosin dye (1^3).Maturated and di¡erentiated in the bone marrow, eosinophils are released at a low rate into the blood circulation, and their levels are only 1^2% of the total peripheral leukocytes in healthy subjects (2). Traditionally viewed as killer-e¡ector cells in helminth parasitic infections and as proin£ammatory cells in allergic diseases, eosinophils are known to be attracted from the peripheral blood circulation toward the in£amed tissues, where they can modulate the in£ammatory process by releasing a range of toxic basic proteins, lipid mediators, cytokines, and superoxide anions (1). It is noteworthy, however, that under physiological conditions 95% of the total eosinophil population are found marginated in submucosal region of the respiratory, gastrointestinal, and genitourinary tracts (2). A recent study has demonstrated a causative relationship between eosinophil recruitment and class l^restricted T-cell selection in the thymus, indicating an immunomodulatory role for eosinophils under nonpathological conditions (4).