The CD3 complex, also known as the T3 complex, is a multimeric protein complex composed of four different polypeptide chains: epsilon (ε), gamma (γ), delta (δ), and zeta (ζ). These chains assemble and function as three pairs of dimers: εγ, εδ, and ζζ. The CD3 protein complex is a defining feature of the T-cell lineage, thus anti-CD3 antibodies can be effectively used as markers for T cells.