The CD3 complex is a component of the T cell receptor (TCR), consisting of CD3γ, CD3δ, CD3ε, and ζ chains, forming the TCR-CD3 complex primarily expressed on the surface of T cells. The structure of CD3 enables it to play a central role in T cell activation and signal transduction by initiating T cell responses through recognition of antigenic fragments presented on antigen-presenting cells. CD3 activates multiple signaling pathways such as NF-κB, MAPK, and PI3K pathways, regulating T cell proliferation, differentiation, and cytokine release. The expression of the CD3 gene is highly specific to T cells, and its expression levels and functional activity are crucial for T cell immune responses. Mutations or aberrant expression of the CD3 gene are associated with various immunodeficiency disorders, autoimmune diseases, and T cell-related cancers.