The cells lining the tonsillar crypts express PD-L1 (Programmed Death Ligand-1) at their surface, while the majority of site-infiltrating CD8+ T cells express high levels of PD-1 (Programmed Cell Death-1), which suppresses the excessive activation of T cells and may result in an immune-privileged site for viral infection and tumourigenesis [62]. Here, PDCD1 is linked to viral infectious disease.