TCR (T cell antigen receptor) recognizes foreign antigens and translates such recognition events into intracellular signals that elicit a change in the cell from a dormant to an activated state. TCR is a heterodimer composed of either alpha and beta or gamma and delta chains. The vast majority of circulating T cells (95%) express the alpha/beta heterodimer while roughly 2-5% express the gamma/delta heterodimer. CD3 chains and the CD4 or CD8 coreceptors are also required for efficient signal transduction through the TCR.