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macroglobulin (mak-roh-glob-yoo-lin) 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 any bacteria that enter the bloodstream. See also immunoglobulin.

2. an abnormal form of IgM produced by lymphoma cells or in other plasma-cell disorders.