Antibodies have a well-defined structure. Each antibody has two identical "light" chains and two identical "heavy" chains. Each chain comprises a constant region, i.e., a region that is the same between antibodies of the same class and sub-class, and a variable region that differs between. The antigen-binding region or binding site - complementarity determining region - is in the variable region. The antibody can be cut by proteases into several fragments, known as Fab, Fab', and Fc.