YB-1 is expressed in germinal center B cells but not in mature PCs in activated tonsils. (A-C) Immunohistochemical analysis of YB-1 protein expression in situ in antigen-activated tonsils as secondary lymphoid organs. (A) Centroblasts in the germinal center (*) of the secondary follicle show a strong YB-1 expression (red). The integrated inset in the lower left corner shows a 2-fold magnification of the germinal center with YB-1–expressing centroblasts as indicated by black arrows. (B) Activated extrafollicular B-cell blasts also display expression of YB-1 protein as shown in higher magnification (2-fold) in the inset by white arrows. The gray arrowhead marked the single mature PC that is YB-1⁻. (C) A subepithelial PC accumulation (+) of the same tonsil PCs with a mature PC morphology (inset with a 3-fold magnification) that display no YB-1 expression. Images were acquired as described in “Immunohistochemical analysis.”