How does indirect immunofluorescence of the skin differ from direct immunofluorescence of the skin?

Indirect immunofluorescence studies test for the presence of circulating autoantibodies in the serum, in contrast to direct immunofluorescence studies, which test for the presence of autoantibodies deposited in the skin. The serum from the patient is incubated with an appropriate normal substrate such as monkey esophagus, rat bladder, or human skin. The substrate is incubated with fluorescein-labeled antibodies directed against the antibody in the tissue. The specimen is then examined under a fluorescence microscope. By running this test at various dilutions, the amount of circulating antibody can be determined and reported as a titer. Titers are useful in some diseases, such as pemphigus vulgaris, in determining disease activity and treatment.