Rat IgG2a kappa Isotype Control (eBR2a), FITC, eBioscience?. The isotype of a primary antibody and the application it is being used in 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 commonly used in flow cytometry, they are useful in other applications such as chromatin immunoprecipitation (ChIP), immunohistochemistry,