The N-terminal domains of the H and L chains are called the variable regions (V regions), and the rest of the molecule is called the constant region (C region).
The amino acid sequence of the V region varies from antibody to antibody, accounting for the high degree of three-dimensional structural diversity of immunoglobulin chains. Because of this diversity, antibodies bind to a wide range of antigens. (The diversity of antibodies is described later.)

The protease papain cleaves antibodies above the disulfide bonds that connect the two H chains (the hinge region), generating three fragments. The two N-terminal fragments are called the Fab region, and the C-terminal fragment is called the Fc region. The “ab” in Fab stands for “antigen binding.” The “c” in Fc stands for “crystallizable,” because the well-conserved amino acid sequence allows this fragment to crystallize.

Another protease pepsin cleaves antibodies and generates an N-terminal fragment with the hinge region still attached. This fragment is called the F(ab')₂ (fab two prime) region. The antigen-binding site is called Fab' (fab prime) to distinguish it from the Fab region that does not include the hinge region. The pepsin fragment is designated F(ab')₂ because the two Fab' regions remain connected at the hinge region. The C-terminal pFc' region is digested into small fragments by pepsin.

Fractionation and purification of proteins

Immunostaining