A class of antibodies that is made by the body as the initial response to an infection or immunization. If IgM is made in response to an immunization, a booster shot will result in a "switch" from IgM to mostly immunoglobulin G.

IgM forms polymers where multiple immunoglobulins are covalently linked together with disulfide bonds, normally as a pentamer or occasionally as a hexamer. It has a large molecular mass of approximately 900 kD (in its pentamer form). The J chain is attached to most pentamers, while hexamers do not possess the J chain due to space constraints in the complex.