Immunoglobulin M (IgM) is a type of antibody that is produced by the immune system in response to an infection. There are several synonyms for IgM including gamma-macroglobulin and macroglobulin. It is also abbreviated as μm in scientific literature. In addition to IgM, there are four other classes of immunoglobulins: IgA, IgD, IgE, and IgG. Each class has its unique properties and functions in the immune system. IgM is the first type of antibody produced in response to an infection, and it plays a vital role in activating the complement system, which is responsible for fighting infections. Overall, IgM is a crucial component of the immune system and plays a critical role in protecting the body from infections.