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Isotype controls are most commonly used in flow cytometry and immunohistochemistry experiments as negative controls that measure the level of non-specific background signal caused by primary antibodies. Background signal is due to Igs binding to non-specifcally to Fc receptors present on a cell surface (ex. mice antibodies binding to human leukocytes non-specifically in which investigators would want to use a mouse isotype control when working with human cells or tissues).