T-cell receptors (TCRs) have a similar structure to the Fab fragment of an immunoglobulin molecule, they are heterodimers made up two polypeptide chains: a (40-50kDa) and b (35-47kDa), which are linked by a disulphide bond; which makes up the large extracellular variable and constant regions for binding antigen. This a:b arrangement makes up for the majority of antigen recognition as it is present on >95% of peripheral blood T-cells and majority of thymocytes, however a minority of T-cells present an alternative receptor with a different pair of polypeptide chains; g and d, this set of TCRs are rare in the secondary lymphoid tissues and peripheral blood but predominate at epithelial surfaces; as this subtype of TCR is not seen in the anatomical locations which would normally support the classical mechanism of antigen presentation and lymphocyte proliferation, suggesting that g:d TCRs can recognise antigen in an MHC-independent manner. TCR heterodimers (a:b) form a complete, stably expressed antigen-binding domain that is capable of transmitting a signal upon ligand binding, by the constitutive association of multimeric hypervariable regions known as the CD3 complex which associate with the TCR variable domains, the TCR itself does not have intrinsic intracellular signalling motifs, the CD3 does and can thus cause intracellular signal transduction mediated by the immunoreceptor tyrosine-based activation motif(ITAM) sequencesi, which leads to T-lymphocyte activation. The CD3 complex comprises of 5 invariant chains; ?, ?, ?, ?, ?; these chains are organised into 2 heterodimers (g:? and d:? ) and one homodimer (? ?), which have 4 and 6 ITAMs respectively. The 10 copies of this motif within the cytoplasmic domains of the TCR:CD3 complex become phosphorylated on the conserved tyrosine residues when the TCR: peptide(MHC) interaction occurs, this allows a docking site for adaptor molecules which cause the mitogenic effects which result in polyclonal T-cell expansion.