The T cell receptor (TCR) performs the dual role of antigen recognition and signal transduction in T cell immune responses and plays an important role in viral infections, cancer, inflammation, and autoimmunity. The TCR lacks intracellular signaling domains but interacts with the CD3δε, CD3γε, and CD3ζζ subunits, each of which possess intracellular immunoreceptor tyrosine-based activation motifs (ITAMs) for phosphorylation. The FG loop in TCR Cβ and the DE loop in TCR Cα have been shown to interact with CD3γε and CD3δε, respectively, whereas the CC' loop in TCR Cβ has been found to interact with both CD3γε and CD3δε.