ActMCs polarize when in the presence of imDCs. (A) Maximum projection images of BMMCs primed with fluorescent α-DNP IgE (white) and activated with DNP-BSA. Internalized, cross-linked IgE–FcεRI of activated BMMCs alone is dispersed throughout the cell (top); however, direct contact of activated BMMCs with imDCs results in accumulation of IgE-containing vesicles (bottom). Overlays are differential interference contrast from single Z-slice of same cells. (B) Single planes of confocal Z-stack images of fixed cell labeling of either unactivated (top row) or activated (bottom row) BMMC–DC interactions. ActMCs demonstrate depletion of actin at the site of endosome aggregation near the membrane (box, Activated, actin image), as well as positioning of the MTOC to the side of endosome aggregation (Activated, tubulin image). 58% of actMCs show the polarized phenotype, i.e., demonstrate accumulation of endosomes proximal to the MTOC (33 cells counted across three independent experiments). All images are brightness and contrast enhanced. Bars, 10 µm.