Immunoglobulin M (IgM) is one of several isotypes of antibody (also known as immunoglobulin). IgM is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antigen (something in the body that triggers an immune response like pollen or a virus). In humans and other mammals that have been studied, plasmablasts residing in the spleen are the main source for specific IgM production.