Which of the following best describes the principle of the indirect immunofluorescence assay for detection of antibodies produced in Rocky Mountain spotted fever?

• A. Patient serum is applied to a microtiter plate coated with a monoclonal antibody directed against the target antigen. A detection antibody labeled with biotin and directed against the target antigen is added. After addition of a substrate, a color reaction develops indicating presence of the antigen.
• B. Specific antibodies in the serum sample attach to the antigens fixed to a microscope slide. In a second step, the attached antibodies are stained with fluorescein-labeled anti-human immunoglobulin and visualized with the fluorescence microscope.
• C. The serum sample is treated chemically to link the target antibodies to a fluorophore. The labeled sample is applied to a microscope slide to which the antigen has been attached. Following a wash step, the slide is examined for fluores.
• D. The patient serum is mixed with a substrate that changes color when antibodies bind to the antigen; a visible color change indicates antibody presence.

Answer: (B)

Indirect immunofluorescence detects antibodies by a two-step approach: fixed antigen on a slide captures any specific antibodies present in the patient’s serum, and a fluorescently labeled secondary antibody against human immunoglobulin then binds those captured antibodies so you can visualize them under a fluorescence microscope. For Rocky Mountain spotted fever, the slide carries RMSF antigens; if the patient has antibodies to these antigens, they attach, and the fluorescein-labeled anti-human IgG (or appropriate isotype) lights up the binding. This readout is specific to the antibodies in the serum and uses the secondary antibody to generate the fluorescent signal, which is precisely what the indirect method entails.

The other designs describe different formats: one is a colorimetric, plate-based detection without fluorescence; another describes directly labeling the patient antibodies with a fluorophore before applying them; and the last relies on a color change from a substrate rather than fluorescence.