Immunoglobulin M

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Shortened to IgM


1 = IgM monomer

2 = Heavy chains

3 = Light chains

4 =J chain

5 = Disulfide bonds

IgM - B. Catchpole, RVC 2008


Is found in high concentrations in blood plasma (below IgG conc.) and is the major Ig produced, by plasma cells, during the primary immune response


IgM is the primordial antibody and, although a monomer, is secreted as a pentamer (five monomers joined by disulphide bonds with two monomers joined by a J chain). This gives it ten identical antigen binding sites although IgM usually has relatively low affinity for its antigen. Its heavy chain is type mu (µ).


Produced by plasma cells in the spleen, bone marrow and lymph nodes. Since it is around five times larger than "normal" antibody it does not diffuse quickly or enter tissues readily and as such its concentration in extracellular fluid and lymph is very low. It does not cross the placenta even in animals with haemochorial placentas e.g. apes.

IgM is the first class of antibody produced by plasma cells, with production replaced by either IgG or IgE in a process known as class-switching once the secondary immune response is initiated.


IgM mainly functions as a target for complement activation as well as roles in agglutination, opsonisation and virus neutralisation.

Immunoglobulins: Immunoglobulin A, Immunoglobulin D, Immunoglobulin E, Immunoglobulin G & Immunoglobulin M

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