Figure 1: Immunoglobulin G (IgG). IgG consists of four polypeptide chains, two light chains (L) and two heavy chains (H). Each light chain is linked to a heavy chain by a disulphide bridge and thetwo heavy chains are also linked to each other by disulphide bridges in the hinge region of themolecule. Comparison of the peptide sequences of the heavy and light chains was able to differentiate two regions: the variable part representing the binding site for Ag (Fv) and the constant part, which gives IgG its effector properties (Fc). A more accurate analysis of the peptide sequences showed that they contained several counterpart units: 2 for L and 2 for H. For L one can distinguish a variable domain (VL) and a constant domain (CL), and for H a variable domain (VH) and three constant domains (CH1, CH2 and CH3). The combination between a VH domain and the adjacent variable domain VL forms the recognition site for Ag. The antigen binding site is formed by six loops, three on the surface of VL and 3 on the surface of VH, called hypervariable regions or CDRs.