The so-called CD3 antigen, also referred to as the CD3 receptor is the cluster of differentiation counting protein complex that is composed of four polypeptide chains. In mammals, it consists of a gamma chain, a delta- chain and two epsilon chains. Form together with the T-cell receptor (TCR) and the so-called zeta chain of the TCR -CD3 complex, which is used for the activation of T lymphocytes.
CD3 chains are transmembrane proteins that belong to the immunoglobulin superfamily. The transmembrane domain of the CD3 chains is negatively charged and therefore allowed to bind to the TCR. The intracellular portion of CD3 chains contain highly conserved peptide segments, so-called immunoreceptor tyrosine -based activation motif ( ITAM short ), which is part of the CD3 -TCR associated intracellular signaling pathway. After phosphorylation of the intracellular portion of the so-called CD3 ZAP70 protein binds to the receptor, so that the signaling cascade is initiated.