Immunoglobulins are a class of proteins, also known as antibodies. They are antigen receptors of the B cells in adaptive immune response. Immunoglobulins can be found in all extant jawed vertebrate species. As the name implies they are associated with the immune system. Their biological function is to specifically tag foreign materials for destruction and removal by the host.
The structure of immunoglobulin is composed of two light chains and two heavy chains. Protease action on an immunoglobulin can cleave it into two different regions, an Fc part and Fab part in the ratio of 1 to 2. The two Fab parts contain the antigen binding sites, which are identical. The Fc part is recognized by cell surface receptors and is also important in an antibody’s immune function.
Both the light and heavy chains contain constant and variable regions. The variable regions are located at the binding site and vary from every immunoglobulin producing cell. The constant regions make up the rest of the immunoglobulin structure with the light chain being designated either lambda or kappa and the heavy chain being either a, d, e, g and m. Immunoglobulins are classified into the following classes based on the type of the heavy chain they contain: IgA, IgD, IgE, IgG and IgM. All the immunoglobulin classes are structurally homologous but some are aggregates of the basic structure with either 2 subunits such as some IgAs, or 5 subunits such as all IgMs bound together by a joining chain.
The V-DOMAIN of the IG and TR are encoded by V-(D)-J rearrangements. Thus, the VH of an IG heavy chain corresponds to a V-D-J-REGION, whereas the V-KAPPA or V-LAMBDA (or V-IOTA for fishes) of an IG light chain corresponds to a V-J-REGION. A V-DOMAIN comprises about 100 amino acids and is made of nine antiparallel beta strands (A, B, C, C', C'', D, E, F, and G) linked by beta turns (AB, CC', C''D, DE, and EF) and three loops (BC, C'C'', and FG), forming a sandwich of two sheets [ABED] [GFCC'C'']. The sheets are closely packed against each other through hydrophobic interactions, giving a hydrophobic core, and joined together by a disulfide bridge between a first highly conserved cysteine (1st-CYS) in the B strand (in the first sheet) and a second, equally conserved cysteine (2nd-CYS) in the F strand (in the second sheet).
Immunoglobulins related literatures
1. Lefranc M P. Immunoglobulins: 25 years of immunoinformatics and IMGT-ONTOLOGY[J]. Biomolecules, 2014, 4(4): 1102-1139.