What are the different types of Immunoglobulins found in Humans?

Immunoglobulins, also known as antibodies, are the large proteins that are produced by the B cells of the immune system. They are used, primarily, to find and neutralize bacteria and viruses (among other things). Antibodies function by identifying specific parts of foreign bodies known as antigens. Antibodies are shaped, generally, like a large Y. The bottom of the Y is the part that is recognized as self and the top part of the Y is highly variable, which allows each antibody to be specific to one particular antigen. This is what leads to the high specificity and variability of the adaptive immune system. Humans (and other mammals) have five different types of antibodies (isotypes), all of which play different roles in the immune system. These five isotypes are immunoglobin A (IgA), IgD, IgE, IgG, and IgM. Of these, there are two subtypes of IgA and four subtypes of IgG.

IgA is found mainly in areas which contain mucosa. These areas include the gut, the respiratory tract, and the genital tract. IgA is produced at a very high rate and between three and five grams of IgA is secreted by the body each day. IgA is the primary constituent of mucus and is also found in tears and saliva.

IgD makes up a relatively small percentage of all antibodies within the body. This isotype is usually co-expressed with IgM on the plasma membranes of B cells. It can also be found within blood serum, but in minuscule amounts. IgD acts as a signal mechanism that alerts B cells to activate. Once activation occurs, IgM becomes the only isotype that is expressed on those activated B cells. In addition, it is believed that IgD helps to stimulate the activation of mast cells and basophils.

IgE is the antibody that is primarily responsible for allergies. Though it plays a large role in allergies and allergic reactions, its main function is to provide the body with an immune response to parasites. The role IgE plays in allergic reactions is due to binding of the bottom of the Y portion (the Fc region) of the antibody to mast cells and basophils, which then release histamine and various other chemokines and cytokines into the body (which can subsequently cause an allergic reaction by triggering inflammation at the site of infection).

IgG is the isotype that provides the vast majority of the antibody immunity against infections and pathogens. This is also the only antibody that is capable of crossing the placenta and providing passive immunity to the fetus from the mothers immune system. IgG represents around seventy five percent of the Immunoglobulins found in human blood serum. IgG are secreted by B cells of the adaptive immune system in very high amounts. These antibodies are used to bind pathogens (viruses, bacteria, fungi, etc.) as well as various toxins that can be produced by those pathogens. IgG is able to form pentamers and can bind different pathogens at various sites in order to neutralize their action and to immobilize them via agglutination.

IgM is the primary antibody that is found on the surface of B cells and is secreted, with very high specificity, in the form of a pentamer in order to eliminate pathogens. It is primarily used to eliminate pathogens and other infectious agents in the stages of the immune response before IgG is produced.

The various types of immunoglobin isotypes that are found within the human body all play varied and important roles in the immune system. Each isotype operates in a different enough way to provide a wide range of coverage to protect against infectious agents. IgG is primarily secreted for use in circulation. IgM is found on the surface of B cells and is primarily used for additional protection in the early stages of infection. IgE is used to protect against parasitic infection. IgA helps to bind antigens in mucus for release from the body. Finally, IgD is used to signal B cells that there is an infection and that other isotypes of antibodies need to be put into production.

Written By: Alexander Maness, United States (M.S. in Biotechnology)

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Edited by: Rajesh Bihani ( Find me on Google+ )

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