The typical structure of immunoglobulin or antibody is Y shaped and is represented as ${\mathrm{H}}_2{\mathrm{L}}_2$. This means it consists of two heavy chains and two light chains. These chains are polypeptide chains and because of difference in weights one is termed as heavy and other is termed as light. Both these chains are linked by disulphide bonds. Each chain consists of two regions: constant region or FC i.e. fragment crystallizable and variable region or Fab i.e. fragment for antigen-binding region. Thus the variable region consists of antigen-binding site that is a part of heavy and light chains. Antigens are attached to antibodies through non-covalent bonds.