Abstract
Bronchial biopsies from two patients with atopic asthma were analyzed by immunogold labeling to detect the ultrastructural location of interleukin 5 (IL-5). In eosinophils, IL-5 was localized to the electron-dense crystalloid core compartment of the secondary or specific eosinophil granules. Other structures in the eosinophils were unlabeled. No IL-5 was detected in mast cells. Control sections incubated with an irrelevant primary antibody were negative. This study demonstrates that pre-formed IL-5 is stored within the major population of secondary granules in human eosinophils.