Antibodies are also called immunoglobulin, the antigen- binding glycoproteins, which are exclusively synthesized by B-cells and in billions of forms with different amino acid sequences and different antigen binding sites.

The simple antibody structure has two identical antigen-binding sites forming a Y-shaped molecule. These antigen-binding sites are present on the tip of each arm of the Y.

Immunoglobulin has five isotopes with different heavy chains, i.e. IgM has a μ heavy chain, IgE has ε, IgA has α, and IgG has a γ heavy chain.

CDR is complementarity determining regions which impart the diversity in the variable region of both heavy and light chains. These are three small hypervariable regions and remaining part is called framework region.

Edelman discovered that when immunoglobulin treated with betamercaptoethanol, it will fall apart into four chains, i.e. two identical light chain and two large heavy chains. This result was considered to propose a structure of immunoglobulin.