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Immunoglobulin M

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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IgM is a large pentameric structure, whose higher molecular weight tends to confine it within blood vessels.

Like IgG, IgM is able to combine with antigen and activate complement. It is the first immunoglobulin to be produced following antigenic challenge. Unlike IgG, it does not cross the placenta.

Monomeric IgM, on the cell walls of B lymphocytes, is the major antibody receptor for antigen recognition.

In brief (in adults):

  • High IgM (above normal range)
    • elevated IgM levels are a nonspecific marker of inflammation, but can be associated with liver disease

  • Low IgM (below normal range)
    • low IgM levels are commonly nonspecific. Significance of this result depends on the clinical presentation and other laboratory parameters

In detail:

Causes of a raised IgM level include:

  • liver disease
  • infection
  • Waldenstrom's macroglubulinaemia

Causes of reduced IgM levels include:

  • hereditary deficiency
  • acquired deficiency
  • protein-losing syndromes
  • non-IgM myeloma
  • infancy, early childhood


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