Mouse IgM Isotype Control (PFR-03). IgM (Immunoglobulin M) is expressed intracellularly during the early stages of B lymphopoiesis, and then on the surface of more mature B cells in the bone marrow and peripheral B cells. The isotype of a primary antibody and its application can result in background staining. Primary antibody background noise can be caused by binding to Fc receptors on target cells; by non-specific interactions with cellular proteins, carbohydrates, and lipids; or by cell autofluorescence. Isotype control antibodies can act as negative controls to help differentiate non-specific background signal from specific antibody signal because they have no relevant specificity to a target antigen. While isotype controls are most comm