Biopsy of a vaccine-site reaction. A biopsy of vaccine-site reactions was obtained, fixed in formalin, and stained with hematoxylin and eosin. To further characterize the nature of the cellular infiltrate, immunohistochemical staining for CD8⁺ and CD1a was performed. (A) A representative example of a vaccine site reaction biopsy is shown, demonstrating a dense mononuclear cell infiltrate at the vaccine site. (B) A representative example of immunohistochemical staining is shown, demonstrating an infiltrate of CD8⁺ T cells (left) and CD1a⁺ immature DCs (right) at the biopsy site.