Analysis of μ chain aggregation by electron microscopy. Ultrathin cryosections of B cells from patients 1 and 7 were processed for postembedding immunogold labeling with anti-μ mAb followed by incubation with secondary antibodies conjugated to 6 nm colloidal gold. Bold arrows indicate gold-labeled μ chains in a megavesicle for patient 1 and in a small vesicle moving toward the cell surface for patient 7. Dotted arrows indicate gold-labeled μ chains in the ER in both patients. Bars = 200 nm. Original magnification, × 20 000.