Immunoglobulin M (IgM) is one of several isotypes of antibody (also known as immunoglobulin) that are produced by vertebrates. IgM is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antigen. In humans and other mammals that have been studied, Plasmablasts residing in the spleen are the main source for specific IgM production. IgM interacts with several other physiological molecules: IgM can bind complement component C1 and activate the classical pathway, leading to opsonization of antigens and cytolysis; IgM binds to the polyimmunoglobulin receptor (pIgR) in a process that brings IgM to mucosal surfaces, such as the gut lumen and into breast milk. This binding depends on J chain; Two other Fc receptors that bind IgM—Fcα/µ-R and Fcµ-R -- have been detected. Fcα/µ-R, like pIgR, binds polymeric IgM and IgA. Fcα/µ-R can mediate endocytosis, and its expression in the gut suggests a role in mucosal immunity. Fcµ-R (formerly known as Toso/Faim3) binds IgM exclusively and can mediate cellular uptake of IgM-conjugated antigen.