IgM (“Immunoglobulin M”) is the second or third most abundant antibody in circulation (after IgG and often, IgA). This antibody is composed of five individual antibody “monomers” bound together by disulfide bonds. Since each individual antibody has two binding sites, each IgM has a total of 10 places to bind antigen. As a result, the antibody is really good, in general, at agglutinating RBCs and fixing complement. Fortunately, most blood group antibodies that are IgM do not react well at body temperatures, so in most cases, IgM blood group antibodies (as in the Lewis, I, and P1PK/GLOB systems, and part of the MNS system) are not important for us in blood banking, as they are not clinically significant. Of course, ABO antibodies in blood groups A and B are primarily IgM, and they react very well at body temperature and are highly significant.