Immunohistochemical analysis of expression of B220, IgM, and IgD in the spleen of 3-week-old wild-type and F/A-2^+/+ mice. Serial sections of wild-type (left column) and F/A-2^+/+ (right column) spleen were stained for the expression of CD45R (B220; a and b), IgM (c and d), or IgD (e and f). The B220⁺ and IgM⁺ B cell layer surrounding the periarteriolar lymphatic sheath is reduced to a small rim of cells. Splenic marginal zones were visualized by staining marginal zone macrophages for the presence of sialoadhesin (SER-4, red in g and h) and staining sinus-lining cells in the marginal zone for the presence of MAdCAM-1 (MECA-367). In the same sections, B220-positive cells are stained blue. Follicles of blue B220-positive B cells within the borders of the marginal zones (*) are easily distinguishable in wild-type (g and i), but are virtually absent in F/A-2^+/+ spleen (h and j). Serial sections were also stained for CD3 (KT-3; k) and sialoadhesin (SER-4; l). CD3-negative areas within the SER-4–positive marginal zone (*) mark B cell follicles. Micrographs shown are representative of six mice examined. Bar, 100 μM.