Immunoglobulin structure

Immunoglobulins are glycoproteins composed of four polypeptide chains (two light and two heavy chains), linked by disulfide bonds that allow the chains to form a bilaterally symmetric immunoglobulin molecule. The N terminus of each chain possesses a variable domain, which, through the use of the hypervariable complementarity-determining regions, binds antigen. Both the heavy and light chains have C-terminal regions. These regions form the constant regions and further define the class and subclass of the antibody.

This region also determines if the antibody light chain will be kappa (k) or lambda (l). There are five classes of immunoglobulins, called IgA, IgG, IgM, IgD, and IgE, based on the structure of their heavy chains. There are two subclasses of IgA and four subclasses of IgG. All immunoglobulins are glycoproteins and contain between 3% and 13% carbohydrate. The carbohydrate portion essentially maintains the structure of the immunoglobulin. Each antibody can exist as two forms: as a circulating molecule or as a stationary molecule attached to the B-cell surface which functions as the B-cell receptor. The stationary form has a hydrophobic transmembrane portion that functions as an anchor. There are only two classes of light chains: kappa and lambda. Each immunoglobulin molecule has only one class of light chain and only one class of heavy chain, although each class of immunoglobulin can have either kappa or lambda light chains. A monomeric antibody, such as IgG, consists of a single immunoglobulin molecule. Polymeric antibodies, such as IgM and IgA, consist of multiple basic units (e.g., IgM antibody consists of 10 light chains and 10 heavy chains). In addition to the polypeptide chains, other structures can be incorporated into the immunoglobulin molecule, including the J chain, also known as the joining chain, which is associated with all polymeric forms of antibody and is a polypeptide that stabilizes the polymer and the secretory piece of IgA. The secretory component of IgA can help to protect the IgA molecule from proteases within the gastrointestinal tract. This secretory component is only found on the secretory form of IgA.