What is Immunoglobulin M (LgM)?

It is the most primitive immunoglobulin and is present in almost all vertebrates. It is about 5 to 10% in total serum immunoglobulins. In general IgM is the first immune globulin class synthesized by babies and is produced in primary response.

In germline cells, the gene segment encoding the p constant region of the heavy chain is positioned first among other constant region gene segments. For this reason mature B cells express IgM in initial stage.

Immunoglobulin M is the largest antibody in human circulatory system and is often referred as macroglobulin because of its high molecular weight (950000 kD). It is secreted as a penatamere, each monomer of IgM have two antigen binding sites.

The monomers are held together by means of disulphide bonds in Fc regions and a “J chain” (a poly peptide chain) linked by disulphide bonds to the Fc region in two different monomers stabilizes the polymer.

The pentameric structure of IgM gives 10 antigen binding sites and the binding sites are projected towards periphery of the molecule. Even though its valence (binding capacity) is 10, it binds with only 5 antigens or less than that. IgM molecules are very efficient in binding with multidimensional antigens and activating complement system (activation of complement system requires its binding with two FC region of antibody).

The half life period of IgM is 5 days and their synthetic rate is 5-8 mg/Kg body weight /day. Due to short life span their concentration in serum is very low (about 15mg/ml).

The carbohydrate content in IgM is about 10-25% and sedimentation coefficient is 19S for pentamer, 7S for monomer.

IgM immunoglobulins cannot cross placenta, but fetus can synthesize IgM immunoglobulins. IgM detection in new born indicates congenital infections such as syphilis, rubella, toxoplasmosis etc. Most of the natural antibodies (eg blood group Abs) are of the IgM type.

Due to its heavy size IgM remains in blood and provides very good defense against infectious agents entered in to the blood. Since IgM is large in size, it cannot diffuse out from blood vessels; hence it is found in very low quantities in interstetium.