Figure 1.

Immune surveillance. Reproduced from Moy et al. ⁴⁶ with permission from Elsevier. T‐cell activation requires three simultaneous signals in order to carry out its anti‐tumour effects. Signal 1 comprises the T‐cell receptor – HLA interaction, with presentation of antigens from the tumour cell. Signal 2 is a summation of costimulatory and coinhibitory signals. These signals must occur in the presence of Signal 3, made up of immune‐activating cytokines, such as IL‐2 or IFN‐γ. Programmed cell death 1 protein (PD‐1) is an inhibitory receptor. Immune evasion can occur at any of these signals (black arrows), impairing the immune system from effectively eradicating malignant cells.