Rat IgM Kappa Isotype Control Antibody [PerCP/Cy5.5] for FC. Isotype controls are primary antibodies that lack specificity to the target, but match the class and type of the primary antibody used in the application. Isotype controls are used as negative controls to help differentiate non-specific background signal from specific antibody signal. Depending upon the isotype of the primary antibody used for detection and the target cell types involved, background signal may a significant issue in various experiments.