CD3 Delta (CD3δ) is a key signal transduction subunit of the T cell receptor (TCR) complex and a member of the immunoglobulin superfamily. It forms the TCR-CD3 complex together with CD3ε, CD3γ, and CD3ζ. CD3δ contains an immunoreceptor tyrosine-based activation motif (ITAM). After the TCR recognizes the MHC-peptide complex, the ITAM is phosphorylated by Src family kinases, recruiting downstream signaling molecules such as ZAP-70 and initiating a cascade of signals leading to T cell activation, proliferation, and cytokine production. Mutations in the CD3δ gene lead to severe combined immunodeficiency (SCID), with patients exhibiting arrested T cell development and severely impaired immune function. Furthermore, anti-CD3 monoclonal antibodies (e.g., OKT3) target the CD3ε/δ complex to inhibit T cell activation and are widely used in the treatment of organ transplant rejection and autoimmune diseases. In immunological research and clinical diagnosis, anti-CD3δ antibodies are used for T cell identification, diagnosis of immunodeficiencies, and exploration of T cell signaling mechanisms.