Immunoglobulin M

Immunoglobulin M
( IgM)


Phylogentically, IgM is the oldest immunoglobulin class. It is the earliest Ig to be synthesized by the fetus beginning by about 20 weeks of age. It constitutes 5-8% of serum immunoglobulins with a normal level of 0.5-2 mg per ml. It has a half life of about of about five day. IgM is the basic antibody that is present on 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. IgM molecules are polymers of five four-peptide subunits, each bearing an extra CH domain. IgM has a molecular mass of approximately 900 kD (in its pentamer form). Because each monomer has two antigen binding sites, a pentameric IgM has 10 binding sites. Typically, however, IgM cannot bind 10 antigens at the same time because the large size of most antigens hinders binding to nearby sites. As with IgA, polymerization of the subunits depend on the presence of the J chain. IgM antibodies are relatively short lived, disappearing earlier than IgG. Hence, their demonstration in serum indicated recent infection.
Most IgM( 80%) is intravascular in distribution.

The unique structural features of IgM appear particularly suited to the biological role of providing protection against microorganisms and other large antigens that have repeating antigenic determinants on their surface.

IgM deficiencies are usually associated with septicemias.


Name: Sumaya Ahmed Role no. 2006 M023