Expression of CD45R (B220), IgM, CD3ε, and MAdCAM in wild-type (WT) and F/A-2+/+ PPs. Histological examination of small intestine occasionally reveals a small PP in ND 21 F/A-2+/+ animals. These PPs are reduced in size and do not protrude from the serosal surface (right column), compared with PPs from wild-type littermates (left column). Nevertheless, immunohistochemical staining for B220-positive B cells (a and b), CD3+ T cells (c and d), and IgM-positive B cells (e and f) revealed the presence of segregated T cell and B cell areas in F/A-2+/+ PPs. Endothelial cells in sinuses lining the PPs normally express MAdCAM-1 (g and h). Sections were counterstained with hematoxylin. Bar, 200 μM.