The basic structure of an antibody consists of two identical “heavy” polypeptide chains paired with two identicalshorter “light” chains, forming an approximate, flxible Yshape (Fig. 1). The chains are subdivided into domainsconsisting of approximately 110 amino acids, which arelinked by a variable number of disulfie bonds, giving atotal molecular mass of approximately 150 kDa.IgG antibodies, the most abundant class of antibody inserum, consist of one Y unit. The IgG subclasses differmainly in the number of inter-heavy chain disulfie bonds.The CH and CL domains are relatively constant in sequence