For Western blotting, when using an HRP-labeled goat anti-rabbit secondary antibody at a 1:5000 dilution, what should I do if the background is still high?

High background is sometimes not due to the antibody itself, but rather the substrate and handling. ECL that is too sensitive or overexposure can cause uniform background; switch to a low-background substrate or reduce the amount of substrate used. After PVDF membrane transfer, avoid complete drying, and ensure sufficient methanol activation; the luminescent solution must cover the membrane evenly, and the membrane should not be left for too long. The box used for secondary antibody incubation must be clean and not shared with blocking solution. Wear gloves during handling and use flat-tip forceps to grip the edges to reduce contamination on the protein side. When these small details add up, they are more effective than simply changing the antibody.

Blocking must be sufficient, but do not rely on only one blocking agent. Routinely use 5% skim milk powder for blocking at room temperature for 1 h. If the background is still high, switch to 5% BSA or a mixture of skim milk + BSA. The blocking solution is best prepared fresh and centrifuged to avoid particles depositing on the membrane. For phosphorylated targets, do not use milk; switching to BSA is more reliable. If necessary, block overnight at 4 °C. After blocking, there is no need to wash for too long; a quick wash with TBST is sufficient. One more thing that is easily overlooked: the blocking solution must cover the entire membrane, with no air bubbles.

It is strongly recommended to prepare a membrane with no primary antibody and only HRP-conjugated secondary antibody. If the background on this membrane is still high, the problem lies in the secondary antibody, substrate, or washing; if there is no background, the nonspecificity comes from the primary antibody/sample itself. If the high background is around the bands, also check whether there are nonspecific bands near the target band; the loading amount can be reduced from 20 μg to 10–15 μg, or the specificity of the primary antibody can be improved. Prepare an additional loading control membrane to examine the overall background distribution, which can help determine whether the problem is local or affects the entire membrane.

If you've optimized dilution, blocking, and membrane washing and the background is still high, switching to pre-adsorbed secondary antibodies is worth considering. Cell lysate composition is complex, and ordinary goat anti-rabbit IgG (H+L) may recognize certain contaminating proteins. Choosing a goat anti-rabbit secondary antibody pre-adsorbed against mouse and human serum can reduce species cross-reactivity and background. Jackson, Invitrogen, Abcam, and others all offer such products; they are somewhat more expensive than standard ones, but stability is usually good. I wouldn't want to give specific catalog numbers arbitrarily—contact your distributor directly, tell them you need low-background WB, and ask them to recommend a pre-adsorbed lot; it's best to first try a small pack on one membrane.

Don't rush to change the primary antibody. A high background at 1:5000 is most commonly caused by an excessive secondary antibody concentration. I would run two gradients, 1:10000 and 1:20000, either at room temperature for 1 h or at 4 °C overnight; the difference in bands and background will be very obvious. In TBST, Tween-20 can be increased to 0.1%–0.2%, and the membrane should be washed at least 3 times, 10 min each, with the shaker speed not too slow. Exposure time should be just enough to visualize weak bands; do not prolong exposure to rescue weak signals. If the background can be reduced this way, simply fix it as the routine ratio.