IgM is synthesized and secreted by plasma cells and is the largest class of immunoglobulins with antibody activity or chemical structure similar to antibody molecules, and can be divided into two subtypes, IgM1 and IgM2. IgM is present in the serum and is the first antibody produced during the development of the individual, and is also the first specific antibody to appear after antigenic stimulation in the humoral immune response, so it can be used in the early treatment and diagnosis of infection. Diagnosis. IgM begins to be produced at 3-5 days of infection, representing that the organism is in acute infection, but the antibody is short-lived, disappears quickly, lasts 2-3 weeks in the blood and cannot pass through the placenta. IgM has a role in lysis of bacteria, activation of complement, immunomodulation and agglutination, and can play a role in the early stages of infection to clear the virus.