n. 1. (immunoglobulin M, IgM) a protein of the globulin series that is present in the blood and functions as an antibody, forming an effective first-line defence against bacteria in the bloodstream. See also immunoglobulin. 2. an abnormal form of IgM (see paraprotein) produced by *lymphoma cells or in other plasma-cell disorders, such as multiple *myeloma.
Health Encyclopedia