Antibodies, also known as Immunoglobulins ( Ig ), are gamma globulin proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses. Antibodies can come in different varieties known as isotypes or classes. In placental mammals there are five antibody isotypes known as IgA, IgD, IgE, IgG and IgM. They are each named with an "" Ig "" prefix that stands for immunoglobulin, another name for antibody, and differ in their biological properties, functional locations and ability to deal with different antigens. IgM is expressed on the surface of B cells and in a secreted form with very high avidity. It eliminates pathogens in the early stages of B cell mediated immunity before there is sufficient IgG. IgM is by far the physically largest antibody in the human circulatory system. It is produced after an animal has been exposed to an antigen for an extended time or when an animal is exposed to an antigen for the second time. IgM forms polymers where multiple immunoglobulins are covalently linked together with disulfide bonds, mostly as a pentamer but also as a hexamer. Because each monomer has two antigen binding sites, a pentameric IgM has 1 binding sites. Typically, however, IgM cannot bind 1 antigens at the same time because the large size of most antigens hinders binding to nearby sites. Due to its polymeric nature, IgM possesses high avidity, and is particularly effective at complement activation.
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