_Immunoglobulin M is a basic antibody that is produced by B cells.
_It is the primary antibody against A and B antigens on red blood cells.
_IgM is by far the physically largest antibody in the human circulatory system. So it cannot diffuse well, and is found in the interstitium only in very low quantities. IgM is primarily found in serum; however, because of the J chain, it is also important as a secretory immunoglobulin.
_It is the first antibody to appear in response to initial exposure to antigen.
Structure and function IgM forms polymers where multiple immunoglobulins are covalently linked together with disulfide bonds, mostly as a pentamer but also as a hexamer.
_It is the first immunoglobulin expressed in the fetus (around 20 weeks) and also phylogenetically the earliest antibody to develop.. IgM antibodies do not pass across the human placenta (only isotype IgG).
=> These two biological properties of IgM make it useful in the diagnosis of infectious diseases. Demonstrating IgM antibodies in a patient's serum indicates recent infection, or in a neonate's serum indicates intrauterine infection (e.g. congenital rubella).
_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 allow 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 on bacteria or perhaps also on plant materials.
_Due to its polymeric nature, IgM possesses high avidity, and is particularly effective at complement activation. By itself, IgM is an ineffective opsonin; however it contributes greatly to opsonization by activating complement and causing C3b to bind to the antigen.

Key word:
Antibody apprearing earliest - ABO - activating complement system