The TcR cannot bind to soluble antigens but they recognize antigenic peptides bound to MHC molecules (i.e. class I or class II). Even after TcR binds MCH-peptide complex, it cannot transmit optimal signal necessary for T cell activation. Intracellular signalling of T cells requires non-covalent association of TcR with cell surface CD3 complex (). The CD3 complex consists of four transmembrane peptides designated γ, δ, ε, and ζ. The CD3 complex itself does not recognize the antigen and does not have variable domains. However, the CD3 complex transmits the biochemical signals generated by the TcR/antigen/MHC interaction on the surface that lead to lymphocyte activation.