Human IgG consists of four subclasses (1-4) that can be recognised by antigen differences in their heavy chains. Each subclass has different biological and physiochemical properties. The IgG subclass may be preferentially produced in response to different antigens.
The Fab fragment (fragment antigen binding) is the antigen region of the IgG molecule. It is composed of one constant and one variable domain of each of the heavy and the light chain.