The CD3 (cluster of differentiation 3) T-cell co-receptor helps to activate the cytotoxic T-Cell. It consists of a protein complex and is composed of four distinct chains. In mammals, the complex contains a CD3gamma chain, a CD3delta chain, and two CD3epsilon chains. These chains associate with a molecule known as the T-cell receptor (TCR) and the zeta-chain (zeta-chain) to generate an activation signal in T lymphocytes. The TCR, zeta-chain, and CD3 molecules together constitute the TCR complex.
