Isotype Controls
Isotype control antibodies are primary antibodies which lack target specificity. Unlike a primary antibody, which targets a certain immunogen, isotype controls will measure the level of non-specific background signal which may come about from use of the primary antibody. They act as a type of negative control, and usually bind Fc receptors on the cell surface.
By including an isotype control antibody in an experiment, researchers can determine if any observed signal is due to specific binding of the primary antibody or non-specific binding of the antibody. This is particularly important when studying weak or low abundance targets, where non-specific binding can significantly impact the results of the experiment.