Various immune checkpoint receptors inhibit effector T cell function and boost T reg cell function. Checkpoint receptors such as CTLA-4, PD-1, and LAG-3 are expressed on activated effector T cells, but are constitutively expressed on T reg cells. In the presence of cognate ligands for these receptors, effector T cell function is diminished, whereas T reg cell function and/or proliferation are enhanced. In this figure, the strength of T cell function and/or proliferation is proportional to the size of the cell.