In indirect immunohistochemistry staining, what is added to tissue sections containing known antigens to test for antibodies to those antigens?

• A. The patient’s serum
• B. A labeled antibody of known specificity
• C. Chromogen
• D. Counterstain

Answer: (A)

In indirect immunohistochemistry used to detect antibodies in a patient’s serum, you apply the serum itself to tissue sections that contain known antigens. If the serum contains antibodies that recognize those antigens, they will bind to them on the tissue. This binding is then revealed with a secondary antibody that recognizes human immunoglobulin and is enzyme-labeled to produce a visible signal. So the reagent added to test for antibodies against the known antigens is the patient’s serum.

Chromogen and counterstain are visualization aids—they help show where binding has occurred and provide tissue contrast, but they don’t introduce or test for the antibodies themselves. A labeled antibody of known specificity would be used to detect a specific antigen in the tissue, not to detect antibodies in the serum.