IgM

IgM in the glossary for Waldenström's macroglobulinemia

The largest of several types of immunoglobulin (antibody) produced by vertebrates. The first antibody produced in response to initial antigen exposure.

Immunoglobulin M (IgM) antibodies are initially expressed during B cell ontogeny and are the first antibodies secreted following exposure to foreign antigens. The secreted IgM multimer has either 10 (pentamer) or 12 (hexamer) antigen binding domains consisting of paired µ heavy chains with four constant domains, each with a single variable domain, paired with a corresponding light chain. Although the antigen binding affinities of natural IgM antibodies are usually lower than IgG, this polyvalency allows for high avidity binding and efficient engagement of complement to induce complement-dependent cell lysis. The high avidity of IgM antibodies renders them efficient at binding antigens present at low levels, and non-protein antigens, for example, carbohydrates or lipids present on microbial surfaces.¹