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B lymphocytes are a type of immune cell which are particularly important in combating bacterial infections. These cells produce very large amounts of antibodies when activated by the presence of antigen from an invading microorganism, but do not all produce the same types of antibody. There are five different antibody types, which are referred to as antibody isotypes. The antibody isotype produced during an immune response usually depends on the type of microorganism which is invading the body.
The five antibody isotypes are known as IgG, IgA, IgE, IgD, and Ig M. The “Ig” prefix is short for immunoglobulin, which is another word for antibody. Each antibody isotype has a different function in the immune system. While the overall structure of all the isotypes is very similar, there are some differences. The main distinction lies in the types of cell receptors which each antibody isotype can bind to, allowing each to activate different parts of the immune system.
The antibody isotype which is present in the greatest abundance in the body is IgG. Up to 75% of antibodies in the blood are of this type. The main function of IgG is to provide protection against reinfection with microorganisms the immune system has previously encountered. One way it does this is by triggering the complement cascade, a series of immune protein reactions which results in the widespread death of invading bacteria.
Two other antibody isotypes, called IgA and IgE, have similar functions to IgG. These three isotypes are able to form chemical bonds with proteins on the surfaces of bacteria and viruses. The microorganisms become coated with antibodies, and these are recognized and destroyed by other cells in the immune system.
IgA and IgE have some additional functions. IgA is highly concentrated in mucosal linings, which are otherwise unprotected by the immune system. IgA is present in the respiratory tract, gut, vagina, and anus. In addition this antibody isotype is secreted in breast milk, thus providing breast-fed infants with immunity to microorganisms their mothers have previously been infected with. IgE is involved in triggering the immune system to attack parasites which infect cells, and is also involved in triggering allergic reactions.
IgM is involved in the initial immune response to an infection, and is one of the first antibody isotypes produced by B lymphocytes when they first become activated. IgM is also capable of triggering the complement cascade. IgD is known to be involved in the early life cycle of B lymphocytes, but has no other known roles.