IgM

Clinical significance

IgM in normal serum is often found to bind to specific antigens, even in the absence of prior immunization. For this reason, IgM has sometimes been called a "natural antibody". This phenomenon is probably due to the high avidity of IgM that allows it to bind detectably even to weakly cross-reacting antigens that are naturally occurring. For example, the IgM antibodies that bind to the red blood cell A and B antigens might be formed in early life as a result of exposure to A- and B-like substances that are present in bacteria or perhaps also in plant materials.