Antibodies (belong to immunoglobulins family) are proteins which are produced by the immune system in response to foreign materials [6]. The most common of these foreign molecules are invading microorganisms such as viruses and bacteria.Antibodies either render them inactive or prepare them for destruction. The proteins of the antibody family are highly developed with the capacity to bind to particular ligands. The antibody family consists of five subdivisions: IgG, IgM, IgA, IgE, and IgD. An antibody recognizes the target, which is generally called an antigen, with a high specificity. Antibodies are Y-shaped molecules (as seen Fig. 5.18). It has been identified that antibodies have two identical binding sites. These binding sites can conform to a small portion of the antigen. The antibody binding sites are formed from several loops of polypeptide chains, which are connected to the end of the protein domains. These protein domains are made of four polypeptide chains, two of which are identical heavy chains and the other two are identical light chains.These chains are all held together by disulfide bounds [6].The amino acids in the binding sites can be changed by mutation without altering the domain structure of the antibody. A large number of antibody binding sites can be formed by changing the length and the amino acid sequence of the loops.