Cluster of Differentiation 3 (CD3) is a pivotal multimeric T cell co-receptor complex essential for adaptive immune responses. Composed of epsilon (ε), gamma (γ), delta (δ), and zeta (ζ) polypeptide chains forming three distinct dimers (εγ, εδ, ζζ), CD3 non-covalently associates with the T-cell receptor (TCR) to establish the critical TCR/CD3 receptor complex on T cell surfaces. Structurally, CD3 proteins belong to the immunoglobulin superfamily, featuring a transmembrane domain, an N-terminal extracellular region with immunoglobulin-like domains, and cytoplasmic tails embedded with immunoreceptor tyrosine-based activation motifs (ITAMs). These ITAMs are fundamental for initiating signal transduction upon T cell receptor engagement, thereby activating both CD4+ T helper and CD8+ cytotoxic T cells. As a defining marker of the T cell lineage, CD3's central role in T cell activation makes its modulation, via stimulating or blocking antibodies, a promising therapeutic avenue for conditions like organ transplant rejection and autoimmune diseases.