B lymphocytes produce antibodies to fight bacterial infections, with five different antibody isotypes: IgG, IgA, IgE, IgD, and IgM. Each isotype has a different function, with IgG being the most abundant and providing protection against re-infection. IgA and IgE have similar functions, with IgA being highly concentrated in mucous membranes and secreted in breast milk, while IgE is involved in attacking parasites and triggering allergic reactions. IgM is involved in the initial immune response, while IgD has no known role.
B lymphocytes are a type of immune cell that is especially important in fighting bacterial infections. These cells produce large amounts of antibodies when activated by the presence of an invading microorganism’s antigen, but not all produce the same types of antibodies. There are five different types of antibodies, called antibody isotypes. The isotype of antibody produced during an immune response usually depends on the type of microorganism that is invading the body.
The five isotypes of antibodies are known as IgG, IgA, IgE, IgD, and IgM. The prefix “Ig” 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 isotypes is very similar, there are some differences. The main distinction lies in the types of cell receptors that each antibody isotype can bind to, allowing each to activate different parts of the immune system.
The antibody isotype present in the greatest abundance in the body is IgG. Up to 75% of the antibodies in the blood are of this type. The main function of IgG is to provide protection against re-infection from microorganisms that the immune system has previously encountered. One way to do this is to trigger the complement cascade, a series of immune protein reactions that result in the widespread death of invading bacteria.
Two other antibody isotypes, called IgA and IgE, have similar functions to IgG. These three isotypes are capable of forming chemical bonds with proteins on the surface of bacteria and viruses. The microorganisms are coated with antibodies and these are recognized and destroyed by other cells of the immune system.
IgA and IgE have some additional functions. IgA is highly concentrated in the mucous membrane linings, which would otherwise not be protected by the immune system. IgA is present in the respiratory tract, intestines, vagina and anus. In addition, this antibody isotype is secreted in breast milk, thereby providing breastfed babies with immunity to microorganisms with which their mothers were previously infected. IgE is involved in triggering the immune system to attack parasites that 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 isotypes of antibodies produced by B lymphocytes when they are first activated. IgM is also capable of triggering the complement cascade. IgD is known to be involved in the first life cycle of B lymphocytes, but has no other known role.
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