IgM immunoglobulin is the first antibody to respond to an antigen or initial infection and makes up about 10 percent of antibodies in serum. IgM is relatively large in size and has a pentamer structure of five monomers bonded together by a joining (J) chain. This chain is a polypeptide. Because of its size, IgM remains in blood vessels and not in tissue fluids. IgM responds to the ABO blood group antigens and enhances the effectiveness of phyagocytic cells. When initial exposure to an antigen occurs, IgM antibodies are the first to appear.
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