The clone M--2E6, a mouse monoclonal antibody selectively recognizes the epitope present in the Fab arms (anti- Fdμ) of the surface monomeric IgM molecule on B cells. It reduces the growth of the neoplastic B cells through its intrinsic crosslinking property in vitro. IgM is the first immunoglobulin expressed in the human fetus at around 20 weeks. It plays various important immunological roles in protecting host from the viral and bacterial infections as well as from the autoimmune diseases.