What is the purpose of including a positive control in immunohistochemistry?

• A. To determine tissue fixation quality.
• B. To calibrate the microscope.
• C. To confirm that staining procedures and antibodies are working correctly.
• D. To identify non-specific binding.

Answer: (C)

The main idea is to confirm that the staining procedure and the antibody are functioning correctly. In immunohistochemistry, a positive control is a tissue sample known to express the target antigen (or a prepared section designed to yield a positive signal) that is processed in parallel with test samples. When the positive control shows the expected staining, you can trust that the antibodies, reagents, antigen retrieval, incubation times, and detection steps are working properly. If the positive control fails to stain as expected, you have a technical problem to troubleshoot before interpreting the test sections, since a negative result in the test sample could be due to a run-wide issue rather than absence of antigen. This purpose is distinct from using a negative control to assess non-specific binding or from evaluating tissue fixation quality, which are addressed by separate controls and assessments.