In immunohistochemistry, what is the primary purpose of including a positive control?

• A. To confirm that staining procedures and antibodies are functioning correctly.
• B. To demonstrate tissue viability.
• C. To assess staining intensity.
• D. To adjust slide thickness.

Answer: (A)

A positive control in immunohistochemistry is used to verify that the staining procedures and antibodies are functioning correctly. By running a tissue sample that is known to express the target antigen alongside test samples, you confirm that antigen retrieval worked, the antibody bound specifically, and the detection system produced a visible signal. When the positive control stains as expected, you can trust that a lack of staining in test tissue reflects true absence of the antigen rather than a technical failure. If the positive control fails, it signals issues with reagents, incubation conditions, or equipment that need troubleshooting before interpreting test results. This concept is distinct from tissue viability (not relevant for fixed IHC), staining intensity (which can vary and requires proper controls to interpret), and slide thickness (affects section quality but isn’t the purpose of a positive control).