Isotype controls are negative controls that indicates the amount of background signal of the primary antibody. An isotype control would be either myeloma-derived antibodies of unknown specificity, or raised against an antigen that is presumed not to be present in the cells being studied.
Ideally, the isotype control should match the species, the heavy chain (IgA, IgG, IgD, IgE, or IgM), subclass and light chain (kappa, lambda) class of the primary antibody. If using a directly labeled primary antibody, it is important to select an isotype control conjugated to the same fluorochrome or label as the test antibody. Using a fluorochrome with a similar spectrum is not sufficient.