CD3ζ see TCRζ.
The CD3 antigen is a complex of 5 invariable polypeptidic chains: γ, δ, ε, ζ and η, whose molecular weights are respectively 25-28, 21, 20, 16 and 22 kDa. The CD3 chains are clustered in a group of two invariant dimers, γ/ε and δ/ε associated with a variable dimer which consists of ζ homodimers, or ζ /η, or ζ/γ FcR heterodimers (γ FcR being the γ chain of the Fc receptors), or γFcR homodimers. The CD3 is part of a bigger complex which includes the T Cell Receptor (TCR). CD3 complex associated with the TCR is involved in the recognition of peptides bound to the major histocompatibility complex class I and II during the immune response. T cell activation may be induced when a foreign antigen is presented to the TCR through MHC complex. The CD3 antigen is expressed by mature T lymphocytes and by a subset of thymocytes.