FIGURE 23-29 Structure of the T-cell receptor and its co-receptors. (a) The antigen-specific T-cell receptor (TCR) is composed of two chains, the α and β subunits, which are produced by V-J and V-D-J recombination, respectively. The α and β subunits must associate with the CD3 complex (see Figure 23-31) to allow the transduction of signals. The formation of a full TCRαβ–CD3 complex is required for surface expression. The T-cell receptor further associates with a co-receptor, CD8 (light blue) or CD4 (light green), which allows interaction with conserved features of class I MHC or class II MHC molecules, respectively, on antigen-presenting cells. (b) Structure of the T-cell receptor bound to a class II MHC–peptide complex as determined by x-ray crystallography.

[Part (b) data from J. Hennecke, 2000, EMBO J. 19:5611, PDB ID 1 fyt.]