CTLA-4 primarily suppresses T cell activation in lymphoid organs by competing with the T cell co-stimulatory receptor CD28 for binding to B7 molecules on antigen-presenting cells, whereas PD-1 dampens T cell effector function in the periphery via its interaction with PD-L1, which is expressed by tumour cells and some immune cells [53]. Here, PDCD1 is linked to neoplasm.