Although every immunoglobulin has its own unique structure, they are all built from the same basic units. All immunoglobulins are made up of 4 polypeptide subunits, each of which has 2 identical H or heavy chains of 50-70kD and 2 identical L or light chains of 23kD. The heavy and light chains are structured in a bilaterally symmetric Y shape and held together by a combination of covalent inter-chain disulfide bonds and non-covalent interactions. The number of inter-chain disulfide bonds is different in each immunoglobulin molecule.

The heavy chains contain 3 constant regions and 1 variable region. At its N-terminus, each heavy chain forms an antigen-binding domain with a light chain. The two antigen-binding domains, also known as Fab or fragment antigen-binding domains, form the arms of the Y shaped structure. The region at which the arms of the immunoglobulin molecule forms the Y shape is called the hinge region because the molecule has some amount of flexibility at this point.