The CD8A–CD8B heterodimer is a co-receptor expressed on the surface of cytotoxic T lymphocytes, where it binds to MHC class I molecules and enhances T-cell receptor signaling during antigen recognition. This interaction is critical for the activation, proliferation, and effector function of CD8⁺ T cells, which play a central role in antiviral and antitumor immunity. In drug discovery, the CD8A/CD8B heterodimer is an important marker and functional target for immuno-oncology research, as it is widely used to evaluate T-cell responses, assess the efficacy of immune-modulating therapies, and guide the development of drugs that aim to enhance or fine-tune cytotoxic T-cell activity.