PDCD1 and infection: This condition is called immune exhaustion (IE) and it is defined by poor effector function (e.g. reduced IFN-γ production) and a sustained expression of inhibitory receptors (cytotoxic T-lymphocyte-associated protein 4 [CTLA-4], programmed cell death protein 1 [PD-1], and T-cell immuno- globulin domain and mucin domain 3 [TIM-3]), on the surface of T lymphocytes, which prevents optimal control of infections and tumours (70).