IgM (Immunoglobulin M) antibody molecule consisting of 5 base units.
1: Base unit.
2: Heavy chains. A heavy chain is the large Polypeptide subunit of a Protein complex, such as a Motor protein (e
3: Light chains.
4: J chain. J chain is a protein component of the Antibodies IgM and IgA.
5: Intermolecular disulfide bonds.

IgM scheme. Heavy chains are blue; light chains are yellow.

Immunoglobulin M, or IgM for short, is a basic antibody that is present on B cells. Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily B cells are Lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response, which is governed by It is the primary antibody against A and B antigens on red blood cells. An antigen (from antibody-generating) or immunogen is a substance that prompts the generation of Antibodies and can cause an immune response Red blood cells are the most common type of Blood cell and the Vertebrate body's principal means of delivering Oxygen to the body tissues via the Blood IgM is by far the physically largest antibody in the human circulatory system.

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. A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily In Chemistry, a disulfide bond is a single Covalent bond derived from the coupling of Thiol groups IgM has a molecular mass of approximately 900 kD (in its pentamer form). The molecular mass (abbreviated m of a substance, more commonly referred to as molecular weight and abbreviated as MW, is the Mass of one The unified atomic mass unit ( u) or Dalton ( Da) or sometimes universal mass unit, is an unit of Mass used to express Because each monomer has two antigen binding sites, a pentameric IgM has 10 binding sites. A monomer (from Greek mono "one" and meros "part" is a small Molecule that may become chemically bonded to other Typically, however, IgM cannot bind 10 antigens at the same time because the large size of most antigens hinders binding to nearby sites.

The J chain is found in pentameric IgM but not in the hexameric form, perhaps due to space constraints in the hexameric complex. J chain is a protein component of the Antibodies IgM and IgA. Pentameric IgM can also be made in the absence of J chain. At present, it is still uncertain what fraction of normal pentamer contains J chain, and to this extent it is also uncertain whether a J chain-containing pentamer contains one or more than one J chain. ^[1]

Because IgM is a large molecule, it cannot diffuse well, and is found in the interstitium only in very low quantities. Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement Interstitial fluid (or tissue fluid) is a solution which bathes and surrounds the cells of multicellular animals IgM is primarily found in serum; however, because of the J chain, it is also important as a secretory immunoglobulin. Blood plasma is the Liquid component of Blood, in which the Blood cells are suspended

Due to its polymeric nature, IgM possesses high avidity, and is particularly effective at complement activation. In Proteins avidity is a term use to describe the combined strength of multiple bond interactions The complement system is a Biochemical cascade that helps clear Pathogens from an organism

Expression

In germline cells, the gene segment encoding the μ constant region of the heavy chain is positioned first among other constant region gene segments. In Biology and Genetics, the germline of a mature or developing individual is the line (sequence of Germ cells that have genetic material that A heavy chain is the large Polypeptide subunit of a Protein complex, such as a Motor protein (e For this reason, IgM is the first immunoglobulin expressed by mature B cells.

It is also the first immunoglobulin expressed in the fetus (around 20 weeks) and also phylogenetically the earliest antibody to develop. ^[2]

Clinical significance

IgM antibodies appear early in the course of an infection and usually reappear, to a less 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). Congenital rubella syndrome (CRS can occur in a developing fetus of a pregnant woman who has contracted Rubella during her first trimester

Other points

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 in nature. An antigen (from antibody-generating) or immunogen is a substance that prompts the generation of Antibodies and can cause an immune response 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.

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. Agglutination is the clumping of particles The word agglutination comes from the Latin agglutinare, meaning "to glue to Red blood cells are the most common type of Blood cell and the Vertebrate body's principal means of delivering Oxygen to the body tissues via the Blood Blood transfusion is the process of transferring Blood or blood-based products from one person into the Circulatory system of another A blood type (also called a blood group) is a classification of Blood based on the presence or absence of inherited Antigenic substances on the

See also

• Antibody
• IgA, IgD, IgE, IgG

References

External links

• MeSH Immunoglobulin+M