Localization of proMBP and MBP by immunoelectron microscopy. (a) A peripheral blood eosinophil labeled with anti-proMBP J163-15E10. Note the absence of specific labeling. (b) A peripheral blood eosinophil labeled with rabbit anti-MBP. Note that MBP is specifically localized to the granule crystalline core (black arrows). (c) Day-24 IL-5–induced UCC labeled with anti-proMBP. The black arrow shows localization of proMBP predominantly to the periphery of a condensing granule; very little proMBP is present in the interior of that granule. The white arrow points to a condensed granule, which lacks proMBP label. Also, note the presence of a large uncondensed granule densely labeled with proMBP proximal to the nucleus (open white arrowhead). (d) Day-24 IL-5–induced UCC labeled with anti-MBP. MBP is localized primarily to the interior of condensing granules (black arrows). (a, c, and d) Original magnification ×11,000; (b) ×15,000.