What is a direct immunofluorescence assay used to detect? Provide an example.

• A. Direct immunofluorescence detects antigen in tissue using labeled antibodies; example: detecting immune complex deposition in lupus nephritis.
• B. Direct immunofluorescence detects antibodies in serum binding to labeled antigens.
• C. Direct immunofluorescence sequences antibodies.
• D. Direct immunofluorescence measures cytokines in serum.

Answer: (A)

Direct immunofluorescence detects antigen present in a tissue by applying fluorophore-labeled antibodies that bind to that specific antigen, allowing visualization with a fluorescence microscope. This approach is ideal for seeing exactly where an antigen sits within tissue and how it’s distributed. For example, in lupus nephritis you can use labeled antibodies against human IgG or complement to reveal immune complex deposits in the kidney tissue, often appearing as a granular pattern along the glomerular basement membrane. This demonstrates the presence and localization of antigen-antibody complexes directly in the tissue.

The other ideas don’t match this approach: using antibodies in serum bound by labeled antigens describes indirect immunofluorescence, which detects antibodies in blood rather than tissue antigens; sequencing antibodies isn’t what immunofluorescence does; and measuring cytokines in serum is a different type of assay altogether, not immunofluorescence.