SIgM

Secretory immunoglobulin M (SIgM) is the secreted form of the IgM antibody class found in mucosal secretions and, to a lesser extent, in serum. SIgM typically exists as a pentamer composed of five IgM monomers linked by a joining (J) chain. During transport to mucosal surfaces, polymeric IgM binds to the polymeric immunoglobulin receptor (pIgR) on epithelial cells. The receptor–IgM complex is transcytosed to the luminal side, where the receptor is cleaved and a portion of it, the secretory component, remains bound to the IgM, forming secretory IgM.

SIgM is produced mainly by plasma cells in mucosa-associated lymphoid tissues, including the gut and respiratory tract. Its secretion into mucosal surfaces provides a first line of defense against pathogens encountered at these sites. In addition to mucosal immunity, small amounts of SIgM circulate in the serum as part of the humoral immune repertoire.

Functionally, SIgM contributes to immune protection by neutralizing pathogens and toxins, promoting agglutination of microbes, and activating the classical complement pathway through C1q binding. The pentameric structure of IgM gives SIgM high overall avidity, allowing effective immune recognition even when individual binding sites have modest affinity.

Clinical relevance includes the role of IgM in early immune responses and mucosal defense. Defects in IgM production or in the pIgR-mediated transport can impair SIgM presence at mucosal surfaces, increasing susceptibility to infections. Elevated IgM levels in serum can reflect certain monoclonal or polyclonal conditions, though such contexts may not directly describe SIgM in secretions.