In simultaneous staining, the two primary antibodies must be from different species.

• A. True
• B. False
• C. Only if using HRP
• D. Not necessary

Answer: (A)

In simultaneous staining, you rely on secondary antibodies that are specific to the species of each primary antibody to keep the signals from each antigen separate. If the two primaries come from different species, you can use one secondary antibody that recognizes the first species and a distinct secondary that recognizes the second species, each conjugated to a different reporter (color or enzyme). This setup prevents cross-binding, so the signal you see can be confidently attributed to the corresponding primary antibody.

If both primaries were from the same species, the same secondary would recognize both, making it impossible to distinguish which signal belongs to which antigen using standard detection methods. Although there are workarounds—such as using subclass-specific secondaries or directly labeling one antibody—the common, straightforward approach for clear, interpretable dual staining is to use primaries from different species. Hence the statement is considered correct, and the other options don’t fit for typical simultaneous staining workflows.