DuckCorp Dico

Found one definition

1.                 From WordNet (r) 3.0 (2006):
                   
   
   
                   
                     
   IgM
       n 1: one of the five major classes of immunoglobulins; involved
            in fighting blood infections and in triggering production
            of immunoglobulin G [syn: immunoglobulin M, IgM]