How to Choose Primary and Secondary Antibodies for Western Blot (and Validate Them)

Last reviewed: 18 October 2026.

A Western blot is only as reliable as its antibodies. This guide explains how to pick a primary antibody for your target and application, how to choose a secondary antibody that detects it without cross-reacting, how to confirm specificity, and how to titrate antibodies for flow cytometry.

Key points

Choose a primary antibody validated for your exact application (WB, IHC, IF, IP or flow) and your sample species; validation in one application does not transfer to another.
The secondary antibody must be raised against the host species of the primary: a rabbit primary needs an anti-rabbit IgG secondary.
Cross-adsorbed (pre-adsorbed) secondaries have had antibodies to other species’ immunoglobulins removed, which reduces cross-reactivity in multiplex detection and on tissue.
Use HRP or AP conjugates for chemiluminescent or colorimetric blots, and fluorophore conjugates for multiplex fluorescent blots and imaging.
When primaries come from two species, choose secondaries from one host (for example donkey) carrying spectrally distinct fluorophores.
An international working group proposed five pillars for antibody validation: genetic, orthogonal, independent-antibody, tagged-protein expression and immunocapture–mass spectrometry strategies (Uhlén et al., Nature Methods 2016).
Validation must be application-specific; a knockout or knockdown control is the most direct test of specificity in a Western blot.
For flow cytometry, titrate each antibody and pick the concentration with the best stain index, (MFI_pos − MFI_neg) / (2 × SD_neg), rather than the brightest signal.

How do I choose a primary antibody for Western blot?

Application. Check that the datasheet lists Western blotting as validated, with an image showing a band at the expected molecular weight.
Species reactivity. Confirm reactivity with your sample species (human, mouse, rat). Sequence homology alone is not proof of reactivity.
Clonality. Monoclonal and recombinant antibodies recognise one epitope and give consistent lot-to-lot results; polyclonal antibodies recognise several epitopes and can give stronger signal, but vary more between lots.
Epitope and isoform. Check which region the immunogen covers (N-terminal, C-terminal, phospho-site) and whether it distinguishes isoforms or cleaved forms you care about.
Denatured vs native. Western blot detects denatured, linear epitopes; an antibody that works in flow or IP (native protein) may not work in WB, and vice versa.

RayBiotech antibody pages list species reactivity, host, clonality and validated applications for each catalog number, for example the RayBiotech anti-CD86 antibody 144-62163 (rabbit; WB, IHC, IF). Browse the RayBiotech product overview.

How do I choose the right secondary antibody?

Decision	Choose	Why
Target species	Anti-[host of primary] IgG, e.g. anti-rabbit IgG for a rabbit primary	The secondary binds the primary’s constant region
Specificity	H+L for general detection; Fc-specific or subclass-specific (e.g. anti-mouse IgG1) for isotype-selective work	H+L also reacts with light chains shared by other classes
Cross-reactivity	Cross-adsorbed secondaries for multiplexing or tissue from a related species	Removes antibodies that bind other species’ immunoglobulins
Detection	HRP or AP for ECL/colorimetric; biotin for signal amplification with streptavidin; fluorophores for fluorescent WB and imaging	Matches your imager and substrate
Fragment	F(ab’)₂ secondaries where Fc receptors are present (cells, tissue)	Avoids Fc-receptor binding background

RayBiotech offers HRP-, biotin- and iFluor-conjugated secondaries, for example anti-mouse IgG (H+L) HRP 127-10394, anti-mouse IgG (H+L) biotinylated 127-10395 and donkey anti-mouse IgG iFluor 488 150-10530.

How do I minimise secondary antibody cross-reactivity?

Use cross-adsorbed secondaries when detecting two primaries at once or staining tissue from a species related to the secondary’s target.
For two-colour detection, use primaries from different host species and secondaries raised in a third species (for example donkey anti-rabbit plus donkey anti-mouse).
Run a secondary-only control (no primary) on every new blot or stain to reveal background from the secondary itself.
Block with a reagent that does not contain the target species’ immunoglobulin; avoid milk when detecting phospho-proteins or using biotin–streptavidin systems.
Titrate the secondary; too much secondary raises background without improving specific signal.

How do I transfer proteins to a PVDF membrane?

Antibody choice cannot rescue a poor transfer. Electrophoretic transfer from the gel to a membrane, first described by Towbin and colleagues in 1979, is where many weak or patchy blots start.

PVDF or nitrocellulose?

	PVDF	Nitrocellulose
Protein binding	Higher; hydrophobic binding	Good; lower than PVDF
Pre-wetting	Must be activated in methanol first	Wet directly in transfer buffer
Mechanical strength	Strong; tolerates stripping and re-probing	More brittle, especially when dry
Background	Can be higher; needs thorough blocking	Usually lower
Best for	Low-abundance targets, re-probing, sequencing	Routine blots, fluorescent detection with low-fluorescence grades

Which pore size?

Use 0.45 µm for most proteins. Choose 0.2 µm for small proteins and peptides (roughly below 20 kDa), which can pass through larger pores during transfer.

PVDF transfer, step by step

Activate the membrane: soak PVDF in 100% methanol for about 15–30 seconds until it turns evenly translucent, rinse briefly in water, then equilibrate in transfer buffer for several minutes. Once activated, do not let it dry; if it does, re-activate in methanol.
Equilibrate the gel in transfer buffer for a few minutes so it does not shrink or swell during transfer.
Build the sandwich from cathode (−, black) to anode (+, red): sponge, filter paper, gel, membrane, filter paper, sponge. Proteins carry negative charge in SDS and move towards the anode, so the membrane must sit on the anode side of the gel.
Remove air bubbles by rolling over each layer; every bubble leaves a blank spot.
Transfer: wet (tank) transfer in Towbin buffer (25 mM Tris, 192 mM glycine, 20% methanol) is the most reliable for a wide size range; semi-dry and rapid systems are faster. Keep tank transfers cold with an ice pack or a cold room.
Check the transfer with a reversible stain such as Ponceau S before blocking; it confirms even transfer and gives a total-protein loading reference.

Large or small proteins

Large proteins (above about 100–150 kDa): lower methanol to 10% or less, add a small amount of SDS (up to about 0.1%) to the transfer buffer, use a lower-percentage or gradient gel, and extend the transfer time.
Small proteins: use a 0.2 µm membrane, keep 20% methanol, and shorten the transfer to avoid blowing through. Placing a second membrane behind the first shows whether proteins passed through.

How do I confirm antibody specificity in a Western blot?

Uhlén and colleagues (International Working Group for Antibody Validation, 2016) proposed five pillars, applied per application:

Genetic strategies: the band disappears or drops in knockout (CRISPR) or knockdown (siRNA) samples.
Orthogonal strategies: protein levels across samples correlate with an antibody-independent method such as targeted mass spectrometry or RNA expression.
Independent antibodies: two antibodies against non-overlapping epitopes give the same pattern.
Tagged-protein expression: the antibody signal matches a tagged version of the target detected with an anti-tag antibody.
Immunocapture followed by mass spectrometry: the protein pulled down by the antibody is identified as the target.

In practice, also check that the band appears at the expected molecular weight, include positive and negative control lysates, and confirm that signal scales with protein load.

How do I titrate antibodies for flow cytometry?

For the controls and gate order that follow titration, see the flow cytometry gating strategy guide.

Prepare a serial dilution of the antibody (typically 6–8 two-fold steps around the datasheet recommendation).
Stain a fixed number of cells in a fixed volume for each concentration, including an unstained or negative population.
Acquire with identical instrument settings and calculate the stain index for each concentration: (MFI_pos − MFI_neg) / (2 × SD_neg).
Choose the concentration where the stain index peaks or plateaus; higher concentrations usually raise background on the negative population.

Sources

Uhlén M, Bandrowski A, Carr S, et al. A proposal for validation of antibodies. Nature Methods. 2016;13(10):823–827. doi:10.1038/nmeth.3995
University of Iowa Flow Cytometry Facility. Stain index calculation. uiowa.edu
BioLegend. The stain index: what is it and what does it tell you? biolegend.com
RayBiotech product datasheets for the catalog numbers cited above. raybiotech.com
Towbin H, Staehelin T, Gordon J. Electrophoretic transfer of proteins from polyacrylamide gels to nitrocellulose sheets: procedure and some applications. PNAS. 1979;76(9):4350–4354. doi:10.1073/pnas.76.9.4350

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