Immunoglobulin M


Immunoglobulin M

Immunoglobulin M, or IgM for short, is a 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.

tructure and function

IgM forms polymers where multiple immunoglobulins are covalently linked together with disulfide bonds, mostly as a pentamer but also as a hexamer. 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.

The J chain is found in pentameric IgM but not in the hexameric form, perhaps due to space constraints in the hexameric complex. 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. [Erik J. Wiersma, Cathy Collins, Shafie Fazel, and Marc J. ShulmanStructural and Functional Analysis of J Chain-Deficient IgMJ. Immunol., Jun 1998; 160: 5979 - 5989.]

Because IgM is a large molecule, 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.

Due to its polymeric nature, IgM possesses high avidity, and is particularly effective at complement activation.

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. 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. ["Review of Medical Physiology" by William Francis Ganong]

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).

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. 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.

ee also

* Antibody
* IgA, IgD, IgE, IgG

References

External links

*


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Immunoglobulin A — (IgA) is an antibody playing a critical role in mucosal immunity. More IgA is produced in mucosal linings than all other types of antibody combined. [cite journal |author=S Fagarasan and T Honjo |title=Intestinal IgA Synthesis: Regulation of… …   Wikipedia

  • immunoglobulin — im mu*no*glob ulin ([i^]m m[ u]*n[ o]*gl[o^]b [ u]*l[i^]n) n. (1953) any one of a class of globular proteins which are antibodies and are produced by the immune system in animals. Note: The immunoglobulins form a series of related proteins which… …   The Collaborative International Dictionary of English

  • immunoglobulin A — immunoglobulin im mu*no*glob ulin ([i^]m m[ u]*n[ o]*gl[o^]b [ u]*l[i^]n) n. (1953) any one of a class of globular proteins which are antibodies and are produced by the immune system in animals. Note: The immunoglobulins form a series of related… …   The Collaborative International Dictionary of English

  • immunoglobulin D — immunoglobulin im mu*no*glob ulin ([i^]m m[ u]*n[ o]*gl[o^]b [ u]*l[i^]n) n. (1953) any one of a class of globular proteins which are antibodies and are produced by the immune system in animals. Note: The immunoglobulins form a series of related… …   The Collaborative International Dictionary of English

  • immunoglobulin G — immunoglobulin im mu*no*glob ulin ([i^]m m[ u]*n[ o]*gl[o^]b [ u]*l[i^]n) n. (1953) any one of a class of globular proteins which are antibodies and are produced by the immune system in animals. Note: The immunoglobulins form a series of related… …   The Collaborative International Dictionary of English

  • immunoglobulin M — immunoglobulin im mu*no*glob ulin ([i^]m m[ u]*n[ o]*gl[o^]b [ u]*l[i^]n) n. (1953) any one of a class of globular proteins which are antibodies and are produced by the immune system in animals. Note: The immunoglobulins form a series of related… …   The Collaborative International Dictionary of English

  • Immunoglobulin G — (IgG) is a monomeric immunoglobulin, built of two heavy chains γ and two light chains. Each IgG has two antigen binding sites. It is the most abundant immunoglobulin and is approximately equally distributed in blood and in tissue liquids,… …   Wikipedia

  • Immunoglobulin D — (IgD) is an antibody isotype that makes up about 1% of proteins in the plasma membranes of immature B lymphocytes where it is usually coexpressed with another cell surface antibody called IgM. IgD is also produced in a secreted form that is found …   Wikipedia

  • immunoglobulin — immunoglobulin. См. иммуноглобулин. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • immunoglobulin D — n IGD …   Medical dictionary


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.