Antibody Structure and Function

Antibody structure and function
 
What are antibodies?
Antibodies are glycoproteins that specifically bind target antigens. Antibodies are present in body fluids and external secretions.
 
What is an antigen?
Antigens are molecules that induce an immune response through the activation of antigen specific B lymphocytes and/or T lymphocytes. Virtually any molecular structure may provoke an immune response, e.g. proteins, carbohydrates, DNA, lipids etc.
 
Antigenic determinant (epitope)
The molecular structure recognised by the binding site of an antibody molecule or a T cell receptor. Antibodies bind to structures presented on the surface of native bio-molecules. The T cell receptor recognises fragments (e.g. peptides) of bio-molecules presented in association with MHC molecules.
 
Antibody structure.
The fundamental structural features of antibody molecules are exhibited by the IgG isotypes. In its simplest form an individual IgG molecule is composed of two identical light chains and two identical heavy chains,. Examination of the amino acid sequence reveals a repeating motif comprised of ~110 amino acid residues with an internal disulphide bridge – referred to as a homology region. Each homology region folds to give a globular protein structure or domain . A light chain is comprised of two homology regions or domains and an IgG heavy chain of four, . The tertiary structure of a domain is referred to as the immunoglobulin fold. Proteins are identified as members of the immunoglobulin super-family if they have primary sequences that are homologous to immunoglobulin domains. A basic structural feature that is retained in this large and growing family of proteins is the immunoglobulin fold.

The gross structure of the IgG molecule is that of three globular regions linked together by a flexible hinge region, There is an axis of symmetry and two of the globular regions recognise and bind antigen in identical manner. These regions...