The immunoglobin monomer structure is additionally classified by its fragments. Primarily the structure is isolated into two groups, the Fc and Fab regions. The constant domain containing Fc region is the class defining region which has no antigen binding capabilities. It is in the Fab fragment that binds the antigen to the structure. The Fab comprises of one constant and one variable region from each heavy and light chain. The variable region of the antibody can also be referred to as Fv, which constitutes the paratope.