Used as a negative control in immunoassays to distinguish specific antibody binding from non-specific background signals. Commonly applied in flow cytometry, ELISA, and immunohistochemistry to validate the specificity of detection systems when studying immune responses. Helps ensure that observed results are due to target-specific interactions rather than artifacts caused by Fc receptor binding or nonspecific protein interactions. Ideal for experiments involving human IgG4 kappa antibodies where accurate baseline measurements are critical.