Immunoglobulin M

Immunoglobulin M, or IgM for short, is a basic antibody that is produced by B cells. IgM is the first immunoglobulin expressed by mature B cells.  It is also the first immunoglobulin expressed in the fetus (around 20 weeks).

IgM is by far the physically largest antibody in the human circulatory system. It is the first antibody to appear in response to initial exposure to antigen. The spleen is the major site of specific IgM production.

Because IgM is a large molecule, it cannot diffuse well, and is found primarily in serum; it is also important as a secretory immunoglobulin.

IgM antibodies appear early in the course of an infection and usually reappear, to a lesser extent, after further exposure. IgM antibodies do not pass across the human placenta.

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 early life exposure to bacteria or perhaps plant materials.

IgM antibodies are mainly responsible for the clumping (agglutination) of red blood cells if the recipient of a blood transfusion receives blood that is not compatible with their blood type.

 

IgM (Immunoglobulin M) antibody molecule consisting of 5 base units.
1: Base unit.
2: Heavy chains.
3: Light chains.
4: J chain.
5: Intermolecular disulfide bonds.