ImmunoglobulinG1-kappa Fab D1.3 (light chain)
Mus musculus (house mouse)
The immune system of the body is its foremost defense system against disease. The newborn has only a poorly developed immune system and as the body of the newborn is exposed to variety of substances, immune response develops. The external agents that activate the immune system are called antigens and the proteins that the body makes in response to these antigens are the antibodies. The antibodies are proteins called immunoglobulins. Antibodies are very specific, but they may sometimes recognize more than one antigen. By a special technique one can produce antibodies that are really specific against one antigen only and these are called monoclonal antibodies. Immunoglobulins have a typical composition of two types of subunits, the heavy chains and the light chains. A heavy chain combines with a light chain to form a dimer and two such dimers combine to form a whole molecule. The overall shape of this molecule is somewhat like the letter Y. The two arms of the letter Y each consist of a light chain and part of a heavy chain. Both divide into so-called constant and variable domains and the variable domains are commonly called VL and VH. A combination of VL and VH is called an Fab fragment and it carries the recognition site for the antigen.
Here we see the example of such an Fab fragment, from a monoclonal antibody, together with its antigen. The antigen in this case is a protein, lysozyme. The monoclonal antibody in this particular case was made in a mouse to recognize just this lysozyme and nothing else, not even other, similar lysozymes. The lysozyme here is hen egg white lysozyme and lysozymes from other birds which have a histidine residue instead of a glutamine residue in position 121 of the lysozyme chain are not recognized well by this antibody. A close look at the structure immediately shows the reasons for it. It is for understanding such highly specific interactions that such detailed studies are useful.
Protein Data Bank (PDB)
Fischmann, T.O. Bentley, G.A. Bhat, T.N. Boulot, G. Mariuzza, R.A. Phillips, S.E. Tello, D. Poljak, R.J.; "Crystallographic refinement of the three-dimensional structure of the FabD1.3-lysozyme complex at 2.5-A resolution";J.Biol.Chem.; (1991) 266:12915-12920 PubMed:1712773.
author: Arno Paehler