PD1 (Programmed Death Receptor 1, PDCD1, CD279) is an immune checkpoint protein active in T cells that is a target alongside its ligand PDL1 and also CTLA-4 for immunotherapy in lung and other cancers. PD1 is involved in negatively regulating T cell inflammatory activity and, when bound to receptors on tumor cells, can work to subdue tumor suppression by inhibiting the immune response. Targeted inhibition of PD1 itself can therefore function as an anti-cancer therapy by reactivating this response. PD1 is expected to have membranous staining in germinal center associated helper T cells, CD8+ T cells, and Pro-B cells. It is for the identification of subsets of T and B cell lymphomas and nodular lymphocyte predominant Hodgkin lymphomas. References: Alsaab, 2017; Jin, 2011; Francisco, 2010; Fife, 2011; Human Pathol 2008 39(7):1050