The multi-chain T cell receptor/CD3 complex (TCR/CD3) plays a key role in antigen recognition, T cell activation and in consequence in triggering an antigen specific immune response. This process is induced by direct interaction of the TCR receptor with an antigen bound to the major histo-compatible complex on antigen-presenting cells. Upon the structural and functional cooperation of TCR receptor with CD3 complex, the activating signal is transmitted through the cell membrane to the nucleus. The pivotal role in signaling cascade plays CD3-zeta (zeta) chain, which triggers many biochemical events and second messenger activation, leading to the transcriptional factors expression and further T cell proliferation, effector function augmentation and cytokine production.