Antibodies are large Y-shaped proteins produced by B-lymphocytes (a white blood cell sub-group) that are used by the immune system to identify and neutralize foreign objects like bacteria and viruses. As such they act like molecular 'blood hounds' tracking and eliminating foreign pathogens from the hosts body.
Each antibody is specific to a particular molecule, or antigen, that is unique to its target. They have a "Y"-shaped structure (Figure 1) consisting of two long protein chains, the so-called "heavy" chains, each combined with a "light" chain. The upper part of the "Y" (the binding domain) recognizes the structure of foreign molecules, such as those located on the surface of a pathogenic micro organism e.g. bacteria, virus, parasite.
Figure 1: Schematic of a human IgG-Antibody
The binding domain of the antibody contains a structure rather like a lock (the paratope) that is specific to a particular structure on the antigen (the epitope) rather like a key. These configurations allow the antibody and antigen to precisely bind together in such a way that the antibody tags the pathogen for attack by other parts of the immune system, through interactions of the lower part of the "Y" (the constant domain). In some cases the antibody itself can neutralize the pathogen by binding to the antigen.
Recombinant Antibodies
For therapeutic purposes, researchers have traditionally used recombinant antibodies such as monoclonal antibodies. These are generated using the hybridoma technique, immortalizing antibody-producing B lymphocytes of mice immunized with a target molecule to obtain specific antibodies against that particular target. Such "murine" (mouse) antibodies originating from one particular B lymphocyte (monoclonal antibodies) can be used to neutralize degenerated cells.
For human therapeutic purposes, it is necessary to reduce the immunogenicity of the antibody so that it will not be recognized as "foreign" and cause allergic side effects. A substantial reduction of the immunogenicity can be achieved by exchanging the constant domains of the murine antibodies for human domains (chimeric antibodies). A further reduction can be achieved by replacing as many sequences of the binding domains as possible with human sequences (humanized antibodies). Those parts of the antibody involved in the recognition of the target molecule cannot, of course, be altered. The various types of recombined antibodies are shown in the Figure 2.
Figure 2: Types of full length therapeutic antibodies
Antibody Fragments
As the first generation of full sized monoclonal antibodies presented some problems, many of the second generation antibodies have comprised only the variable domains (Fv) as fragments of the antibody. Such fragments, with only the binding domains, can be produced in bacterial and eukaryotic cells. Two approaches can be used: either the Fv fragment alone or 'Fab'-fragments comprising one of the upper arms of the "Y" that includes the Fv plus the first constant domains. These fragments are usually stabilized by introducing a peptide link between the two chains which results in the production of a single chain Fv (scFv). The binding domains can be combined with any constant domain in order to produce full length antibodies or can be fused with other proteins and peptides (Figure 3).
Figure 3: Antibody fragments: Fab, Fv and single chain Fv (scFv)