T-cell co-receptor CD3 is a protein complex composed of four distinct chains. In mammals, the complex contains a CD3γ chain,or a CD3δ chain, and two CD3ε chains, along with the T-cell receptor (TCR) and the ζ-chain (zeta-chain) to form the classical T cell receptor complex,which mediates signal transduction, resulting in T cell activation and proliferation,forming the essential T cell immunity as normal immune responses.