Transfused platelets are mainly localized in the RP and MZ of the spleen. Immunostaining of spleen sections from H2^d BALB/C mice 2 hours after transfusion with 1 × 10⁸ CFDA-SE-labeled H2^b platelets (green) isolated from C57BL/6J mice. (A) Macrophages were detected using F4/80 (red) and B cells were detected using B220 staining (blue). (B) The MZ is delineated using IgM staining (red) for MZB cells and MadCam staining (blue) for sinus-lining cells. (C) Quantification of platelets transfused into different compartments of the spleen. Results are expressed as the number of platelets per μm² (n = 17). (D) Five minutes before sacrifice, mice received a retro-orbital injection of APC anti-mouse CD21/35 (1 μg) to label MZB cells (red) in vivo. The dashed and full lines delineate the RP and follicle areas, respectively. Scale bar, 10 μm. Original magnification x400. The right panels show the magnifications of the areas corresponding to the white squares on the left panels. B Foll, B-cell follicle; MZ, marginal zone; RP, red pulp.