Complex Between HEL Antigen and Anti-HEL Antibody Fab

EPITOPE of HEL Hen Egg-white Lysozyme	PARATOPE of Anti-HEL Fab (L:H)
Complex between HEL and the papain fragment - Fab (L:H chain fragment) - of an anti-HEL antibody.

• The structure of an antigen-antibody immune complex is shown with Hen Egg-white Lysozyme (HEL) antigen bound to a papain-digested Fab fragment derived by from an anti-HEL monoclonal antibody.
• One end of the Fab fragment makes direct contact with the surface of HEL effectively burying glutamine 121 (Gln 121) and several neighboring amino acid residues on the surface of HEL.
• Collectively, these amino acid residues define the specific "antigenic determinant" or "epitope" of HEL as recognized by this particular anti-HEL antibody; other anti-HEL antibodies will generally recognize different antigenic determinants and different epitopes of the same molecule. 
• The amino acid residues of the antibody molecule that make direct contact with antigen define the "antigen combining site" of the antibody.  These make up in the "hypervariable loops" or "complementarity-determining regions" (CDRs) of the antibody heavy and light chains.  The CDR residues any given antibody are generally unique and, collectively, they make up the paratope of an antibody, which is operationally defined as the specific group of H and L chain atoms or CDR residues that make direct contact with the atoms that comprise the epitope.
• Overall, the specificity of an antibody for its antigen is determined by the complementary nature of the epitope and antibody contact residues in terms of shape, charge, and chemistry.
• The structural basis for this antigen-antibody interaction is further explored with a series of JSmol commands entered via the JSmol Console.

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© Duane W. Sears
29 August, 2009