Ultrastructural immunogold localization of interleukin 5 to the crystalloid core compartment of eosinophil secondary granules in patients with atopic asthma

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.