IgM (Serum)

Immunoglobulin M (IgM) is the largest antibody and is the first to be synthesized by B cells in response to an initial (primary) exposure to an antigen. It plays a crucial role in early immune defense, particularly against bacterial and viral infections. Elevated IgM can indicate a recent or ongoing infection, Waldenström's macroglobulinemia (a type of lymphoma producing monoclonal IgM), or certain autoimmune conditions like primary biliary cholangitis. Low levels can suggest an immunodeficiency.