Mice lacking either TRAF2 alone or both cIAP1 and cIAP2 in their B cells are unable to form GCs. (A-C) Mice of the indicated genotypes were immunized with SRBCs and killed on day 7. (A) Flow cytometric analysis of splenic B cells from the indicated mouse lines. Data are from total B cells (B220⁺ gate). Window indicates CG B cells (GL7^hi, Fas^hi). Numbers indicate the proportion of displayed events falling within the associated window. (B) Enumeration of the proportion of splenic B cells that were GC B cells as determined by flow cytometry. Points represent data from individual mice; and lines, the arithmetic means. Error bars represent SD. (C) GC formation in the spleens of immunized mice as detected by immunohistochemistry on spleen sections from mice of the indicated genotypes. Sections were stained for CD3⁺ T cells (red), IgD⁺ B cells (blue), and GCs (GL7⁺, green).