Description: The HIS40 monoclonal antibody reacts with the μ heavy chain of rat IgM. It does not react with other classes of rat immunoglobulin including IgD, IgG or IgA. IgM is expressed intracellularly, during early stages of B lymphopoiesis, and then on the surface of more mature B cells in the bone marrow and peripheral B cells. Fluorochrome conjugated HIS40 is used as a detection secondary for rat IgM primary antibodies.