Flow cytometry is a staple application in basic and clinical research as well as in diagnostics. A fundamental decision in experimental design is whether to use conjugated primary antibodies for direct detection, or unconjugated primary antibodies paired with labeled secondary antibodies for indirect detection. Conjugated primaries are covalently coupled to labels such as fluorescent dyes or protein labels, enabling simple one-step staining protocols. Indirect staining is a more involved procedure, but can offer a degree of flexibility across different applications. Here, we discuss the major differences and unique advantages between the two approaches in the context of flow cytometry.

Ease of workflow

Direct staining with conjugated antibodies offers a simpler workflow than indirect staining. Introducing secondary antibodies adds incubation and wash steps, as well as additional protocol constraints. The secondary antibody must be matched to the appropriate host species and, where relevant, the isotype of the primary. In multiplexed panels, host species assignments must be carefully planned to prevent secondary antibodies from cross-reacting with unintended primaries.

“It adds complexity into the staining protocol as the incubation of the primary and secondary pair must be done separately to the other primary antibodies to avoid cross reactivity,” shares Sharon Sanderson, Ph.D., Flow Cytometry Application Scientist and Product Manager at Bio-Rad Laboratories, on the indirect staining approach. “This requires additional wash steps, which increase sample loss. Additionally, secondary antibody staining conditions must be carefully optimized for each primary–secondary pairing, which can take time.”

The added complexity has downstream implications in clinical applications, where process development and validation are more stringent. Each antibody must be validated for antigen reactivity and titrated on the specific sample type. Any modification to the fluorochrome panel requires re-qualification of all reagents to confirm no adverse impact on data quality. In this context, secondary antibodies, along with their associated controls, can substantially increase the time and labor required for workflow qualification.

High-plex panels

High-plex, multicolor panels represent perhaps the clearest use case for fluorophore-conjugated antibodies. This approach eliminates the need for species- or isotype-specific secondary antibodies for each target, a requirement that compounds in complexity as panel size increases.

“Conjugated antibodies are highly preferred for combining in multicolor panels,” comments Christopher Manning, Associate Director of Flow Cytometry at Cell Signaling Technology. “When using unconjugated antibodies, each must have a unique secondary antibody that does not cross-react with others in the panel. For example, you could use a mouse IgG1, mouse IgG2a, and rabbit IgG antibody together in a panel because each could be detected by a different isotype-specific secondary. But you could not use two mouse IgG1 antibodies in the same panel, as both would be labeled by the same secondary.”

Workflow complexity is further magnified as the panel size grows. “If you are using an unconjugated antibody, you will have to titrate the primary and secondary antibody,” says Sanderson. “Whereas if you use conjugated primary antibodies, there is only one antibody to titrate. It is time consuming making multicolor panel staining panels and adding two additional layers of antibody staining also takes time. The quality of the flow data is only as good as the quality of the sample, and time can have a deleterious effect on the sample.”

Flexibility

Conjugated antibodies, because they can be used so easily with each other, are often more flexible in designing multicolor panels. However, where unconjugated antibodies can shine is application versatility. “Unconjugated antibodies may be used across multiple applications—such as flow cytometry, western blotting, and ELISA—more easily than a conjugated antibody, where the fluorophores may be a limitation,” notes Manning.

This versatility is particularly useful when repurposing an unconjugated antibody already validated against an uncommon target or a highly specific epitope. Such an approach is well-supported by the broad commercial market of conjugated secondary antibodies, which span a wide range of host species, reactivities, isotype/subclass specificities, and reporter labels—including both fluorophore and non-fluorophore conjugates—affording considerable flexibility when designing primary–secondary antibody pairs.

The absence of a pre-attached label also opens the possibility of in-house conjugation when a specific fluorophore is better suited to the panel. An antibody targeting a low-abundance protein, for instance, may benefit from pairing with a brighter fluorophore. Suboptimal antibody–fluorophore pairings can meaningfully impair resolution. For example, one study using anti-CD3 paired with BV570 has been reported to produce suboptimal separation of CD3⁻ and CD3⁺ populations, likely in a panel-specific context.

Beyond secondary antibodies, primary antibodies conjugated to non-fluorescent labels offer an additional layer of flexibility. “One option is to use a biotin labeled primary antibody which is then detected by streptavidin conjugated to a fluorophore, this avoids issues with isotype-specific secondary antibodies,” notes Sanderson.

“Self-conjugation kits are also available and represent a cost-effective and convenient alternative to either custom conjugation or secondary antibodies. Our new TrailBlazer™ antibody labeling kit allows you to conjugate any primary antibody to any of our range of industry leading StarBright Dyes, and we have Lynx and ReadiLink kits to conjugate to conventional dyes,” adds Sanderson.

Sensitivity

Signal amplification is another well-known advantage of indirect staining. Because multiple secondary antibodies can bind to the constant domains of a single primary antibody, the number of fluorophores per antibody–antigen complex is greater than what a single directly conjugated primary can deliver. "Unconjugated antibodies detected with a secondary generally provide brighter signal than a directly conjugated antibody," says Manning.

This enhanced sensitivity can be critical when working with difficult-to-detect targets, such as rare or low-abundance antigens or epitopes with restricted steric accessibility. "For low-abundance targets, a primary plus secondary approach may be necessary to resolve population differences, or signal above background," continues Manning.

Cost

Indirect staining can offer potentially lower upfront reagent costs, since pre-existing unconjugated primaries can be repurposed and paired with relatively inexpensive secondary conjugates, rather than requiring the purchase of a purpose-specific conjugated primary. The ability to mix and match primary–secondary combinations also provides cost-effective flexibility for researchers who frequently revise their experimental panels.

Directly conjugated antibodies are typically more expensive to purchase, and they commit a given target to a fixed fluorescence channel. If panel redesign requires that marker to shift to a different channel, a new conjugated antibody must be procured, adding to reagent costs. That said, the simpler workflow associated with direct staining can offset the higher per-reagent cost over time through reduced optimization burden, shorter experiment turnaround, and lower labor investment.

Bottom line

The choice between conjugated and unconjugated primary antibodies should factor both short- and long-term needs of the project. Indirect staining offers a sensitivity advantage for low-abundance targets and preserves fluorophore flexibility without committing to a fixed detection label. This is most valuable during early-stage assay development or when working with a limited antibody repertoire.

For well-established panels and clinical applications, conjugated primaries can be more pragmatic. They simplify the workflow, reduce cross-reactivity risk, and scale cleanly into high-plex multicolor formats where managing species and isotype compatibility across many unconjugated primaries becomes a genuine logistical burden.

References

Mason, E. Antibody labeling: principles, advantages, and considerations. Biocompare. Published August 10, 2023. Accessed March 2026. https://www.biocompare.com/Editorial-Articles/598800-Antibody-Labeling-Principles-Advantages-and-Considerations/

Estipona, D. A Guide to Secondary Antibodies. Biocompare. Published February 10, 2024. Accessed March 2026. https://www.biocompare.com/Antibodies/8959-Secondary-Antibodies/

Whyte CE, Tumes DJ, Liston A, Burton OT. Do more with Less: Improving High Parameter Cytometry Through Overnight Staining. Curr Protoc. 2022;2(11):e589. doi:10.1002/cpz1.589

Lazarski CA, Hanley PJ. Review of flow cytometry as a tool for cell and gene therapy. Cytotherapy. 2024;26(2):103-112. doi:10.1016/j.jcyt.2023.10.005