This chapter provides a comprehensive overview of immune response activation and regulation, with a particular focus on T cell biology and the phenomenon of T cell exhaustion. The content covers several key aspects of immunology, beginning with an introduction to the basic components of the immune system, including innate and adaptive immunity. It then describes the details of T cell development and explore the process of thymic selection and the role of dendritic cells in antigen presentation and T cell activation. The chapter extensively discusses various T cell subsets, including CD4+ helper T cells, CD8+ cytotoxic T cells, and regulatory T cells (Tregs), detailing their functions and importance in immune responses. A portion is dedicated to T cell signaling, explaining the complex cascades involved in T cell activation, including TCR signaling and co-stimulatory pathways. Immune checkpoints, particularly CTLA-4 and PD-1, are examined in detail to highlight their critical role in regulating T cell function and homeostasis. T cell exhaustion is explored in depth, covering its characteristics, underlying mechanisms, and significance in chronic infections and cancer. The content also addresses the transcriptional regulation of T cell exhaustion, discussing key transcription factors and the heterogeneity of exhausted T cell populations. It examines the impact of various cytokines and other factors in the tumor microenvironment on T cell exhaustion. Finally, the chapter concludes with a discussion on CAR-T cell therapy and the challenges posed by T cell exhaustion in this context.