Immunohistochemical staining of PD-L1 and PD-1 in healthy BM and controls. (A) IHC for PD-L1 showed no staining in healthy or reactive BM. (B) Staining of healthy and reactive BM with antibody against PD-1 is generally negative with a rare lymphocyte showing membranous staining. (C) Strong staining of placental syncytiotrophoblasts is shown with antibody against PD-L1. (D) Staining of tonsil with antibody against PD-1 highlights a subset of T cells in the germinal center, as shown at right. Magnification ×40 in panels A,C-D and ×100 in panel B. Images taken with an Olympus BX41 microscope with Olympus cellSens standard version 1.14 software, and standard color balancing and image sizing using Adobe Photoshop.