CD8A and cancer: The group of Dai reported metal-based nanoparticles for real-time imaging of anti-cancer immunotherapy.134 The authors synthesized bright erbium nanoparticles emitting in the long NIR-IIb window, which spans from 1500 nm to 1700 nm, where tissue background is very low and signal-to-noise ratios can be maximized (Fig. 6b).135 The nanoparticles were conjugated to anti-PD-L1 antibodies for targeting and used in combination with T cell markers (in this case, anti-CD8-derivatized quantum dots) to simultaneously monitor T cell location and T cell activation in mouse models of colorectal cancer.