The histology and immunophenotypic characteristics of intraepithelial B cells in a control tonsil. (A) Control tonsil showing reactive follicular hyperplasia with no expansion of the MZ. The crypt epithelium (Ep) contains numerous intraepithelial lymphocytes. (B) CD27 immunostaining highlights expression by the intraepithelial B cells. (C) Double immunostaining for CD3 in brown (DAB) and CD43 in blue (fast blue) shows many individual blue cells expressing CD43, but not CD3, consistent with intraepithelial B cells. (D) Crypt epithelium stained for κ immunoglobulin light chain (top) and λ light chain (bottom). The intraepithelial B cells are polytypic. Original magnifications, × 40 (A); × 200 (B); × 400 (C); and × 100 (D). Microscope, Olympus BX51; numerical aperture of objective lenses used: × 4, 0.16 mm (A); × 10, 0.40 mm (D); × 20, 0.70 mm (B); × 40, 0.85 mm (C). Stains used: hematoxylin and eosin (A) and immunoperoxide (B-D). Camera, Olympus DP11; acquisition software, Adobe Reader 6.0; and software used for image processing, Adobe Photoshop 5.0.