Subserial sections of the thymus stained with antibodies against cytokeratin (A), CD3 (B), CD45R0 (C), CD45RA (D), TCRGD (E), and CD1a (F). In a thymic lobule, which is largely composed of epithelial cells, the CD3⁺ lymphocytes predominantly express CD45R0 and TCRGD. Many lymphoid cells show delicate membrane reactivity for CD1a (streptavidin-biotin complex immunoperoxidase technique, counterstained with Mayer’s hematoxylin; original magnifications: [A] through [E], ×160; [F], ×400).