The presence of B cells and plasma cells in the normal human thymii. Ten pediatric thymii were stained by hematoxylin and eosin, and by immunohistochemistry for the T-cell-specific marker CD3, the pan-B-cell markers CD79α and CD20, and the plasma cell-specific marker CD138. Representative photomicrographs from a single thymus are shown. The CD20⁺ cells appeared to cluster around the Hassel corpuscles (asterisk), while the CD138-staining plasma cells were seen in the medullary areas (arrow). Micrographs are all × 40, except as noted.