The amino acid N-terminal domains of the heavy and light chains include the antigen-binding site. The amino acids of these variable domains vary between different antibody molecules and are thus known as the variable (V) regions. Most of these differences reside in the hypervariable areas of the molecule and are usually only six to ten amino acid residues in length. When the hypervariable regions in each chain come together along with the counterparts on the other pair of H and L chain, they form the antigen-binding site. This part of the molecule is unique to the molecule and is known as the idiotype determinant. In any individual, 10(6) to 10(7) different antibody molecules can be composed from 10(3) different heavy and light chains of the variable regions. The part of the molecule next to the V region is called the constant (C) region made up of one domain in the light chain (C1) and three or four in a heavy chain (CH). AC1 chain may consist of either two kappa (κ) or two lambda (λ) chains but never one of each. Of all the human antibody molecules, approximately 60%, are κ chains and 40% contain λ chains. Although there are no known differences in the functional properties of κ and λ chains, there are several different types of the CH domain. These differences are reflected in determining the class (isotype) of the antibody and thereby the physiological function of a particular antibody molecule.