T cell activation is an activation process initiated by the binding of MHC-peptide complexes on antigen-presenting cells to TCR, together with co-stimulation signals. After activation, downstream signaling is triggered via the TCR-CD3 complex and co-stimulatory receptors. Pathways including PLC-γ, PI3K and MAPK are activated to induce transcription factors such as NFAT and NF-κB, which regulate cell proliferation, differentiation and cytokine secretion, and mediate adaptive immune responses. It serves as the core link of immune defense, autoimmunity and immunotherapy.