Mouse Anti-Human IgM monoclonal antibody for ELISA, IHC-Fr, ICC, IP, WB, FC. Immunoglobulin M, or IgM for short, is a basic antibody that is produced by B cells. IgM is by far the physically largest antibody in the human circulatory system. It is the first antibody to appear in response to initial exposure to an antigen. The spleen, where plasmablasts responsible for antibody production reside, is the major site of specific IgM production.