Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In immunology, the CD3 T-Cell Co-Receptor (CD stands for cluster of differentiation) is a protein complex and is composed of four distinct chains. In mammals, the complex contains a CD3γ chain, a CD3δ chain, and two CD3ε chains. These chains associate with a molecule known as the T cell receptor (TCR) and the ζ-chain to generate an activation signal in T lymphocytes. The TCR, ζ-chain and CD3 molecules together comprise the TCR complex. The CD3γ, CD3δ, and CD3ε chains are highly related cell surface proteins of the immunoglobulin superfamily containing a single extracellular immunoglobulin domain. The transmembrane region of the CD3 chains is negatively charged (it contains aspartate residues in the transmembrane region), a characteristic that allows these chains to associate with the positively charged TCR chains.