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Shortened to IgM
Is found in high concentrations in blood plasma (below IgG conc.) and is the major Ig produced during primary immune response
Structure
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. It heavy chain is type mu (µ ).
Production
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.
Function
- IgM mainly functions as a target for complement activation as well as roles in agglutination, opsonisation and virus neutralisation.