Immunoglobulin G: variability

When we study the amino acid sequence in different immunoglobulins, we can distinguish constant regions and variable regions:

• The first 108 residues in light chains and the first 119 residues in heavy chains are different for each antibody (they are responsible for their specificity) and form the variable region, V_H + V_L. In this region we can find the antigen binding site. Since there are two chains of each type, there are two identical variable regions and each IgG molecule can bind two antigen molecules.
• The remainder amino acids in each chain are identical for all immunoglobulins in a class (IgG, IgA, IgM, IgD or IgE) and form the constant region, C_H + C_L. Note that it covers the whole F_c domain and half of the F_ab domains.

Highlight the variable region: