Variety of Immunoglobulin G Functions

The diversity of immunoglobulin (Ig) functions may be explained by the peculiar structure of these flexible molecules. Heavy and light Ig chains are composed from compact stable immunoglobulin folds (IgF), or domains belonging to a large protein super family. Proteins composed from IgF have been found in various representatives of animal kingdom even those appeared during the very early stages of evolution [1,2]. Such evolutionary conservation supports the well-known fact that useful structural elements appeared early in evolution later preserved and long-lived. The most important feature of IgF lies in its specific ability to bind molecules of various chemical structures and dimensions from large proteins to small molecules such as peptides and simple carbohydrates. First of Ig interactions are related to the formation of immune complexes (IC) of antibody molecules with antigens at antigen-combining sites. Igs are able to form non-IC with various proteins on sites located on only one domain, or on sites composed from residues on different domains [3,4]. Therefore, IgF acts as a scaffold on which arrays of binding sites are displayed on β strands or on those parts on Ig peptide chains that are connected the strands. One aspect of the Ig interactions outside the antigen binding sites with the formation of non-IC is most important for adaptive immune processes while others aspects are involved in some other functions.