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CD3ε is a subunit of the CD3 complex, which is a critical component of the T-cell receptor （TCR） complex on T cells. Together with CD3γ, CD3δ, and CD3ζ, it forms the TCR-CD3 complex that plays a vital role in coupling antigen recognition to intracellular signaling pathways. This complex is essential for T cell activation, proliferation, and differentiation. CD3ε contains an extracellular immunoglobulin domain, a transmembrane region, and a cytoplasmic tail with an immunoreceptor tyrosine-based activation motif （ITAM）. Upon antigen recognition, the ITAMs in CD3ε are phosphorylated, initiating downstream signaling cascades that lead to T cell activation. Additionally, CD3ε interacts with other signaling molecules like Lck and Nck through its unique motifs, further modulating T cell responses. Its importance in T cell function makes CD3ε a target for immunotherapies, including cancer treatments and autoimmune disease management.