The washing step in IHC directly affects two types of outcomes. One is whether unbound or weakly bound detection reagents can be sufficiently removed. The other is whether specifically formed binding complexes can be stably retained. The differences among PBS, TBS, PBST, and TBST are not limited to their names, but involve the buffer backbone, the presence or absence of surfactant, and their corresponding experimental boundaries. During method optimization, determining the basic buffer system first and then deciding whether Tween-20 should be added is usually more effective than simply increasing the number of washes.

Keywords: IHC; PBS; TBS; PBST; TBST; Tween 20; HRP-DAB; AP chromogenic detection; washing system

1 Functional Positioning of Washing Systems in IHC

1.1 Removal of free reagents

The first task of washing is to remove free and weakly adsorbed primary antibodies, secondary antibodies, polymer amplification reagents, and other detection reagents. If this step is insufficient, diffuse light staining is likely to appear in negative regions, and local residual signal is more likely to occur at section edges, tissue clefts, around luminal spaces, and in stromal regions.

1.2 Retention of bound complexes

The washing solution affects not only removal of unbound reagents, but also retention of already formed antigen-antibody complexes. For low-abundance antigens, weak-affinity antibodies, membrane surface markers, and cell boundary-type localization signals, overly strong washing more easily leads to signal attenuation, reduced numbers of positive cells, or blurred boundaries.

1.3 Pre-chromogenic environment control

Different detection systems do not tolerate buffer components equally. Because the washing solution directly enters the environment before chromogenic development, once the wrong basic buffer backbone is chosen, extending the washing time later often cannot compensate for the compatibility bias of the system itself.

1.4 Interpretive value of background pattern

(1) Diffuse light staining in negative areas is commonly associated with excessive reagent residue.

(2) Darker staining at edges, clefts, and around luminal spaces is commonly associated with insufficient wetting or local residual liquid.

(3) Greater deposition in necrotic regions and blood-rich areas is commonly associated with tissue retention and amplified background.

(4) Uniformly high background across the whole section usually requires simultaneous review of blocking, endogenous enzyme treatment, and chromogen development time.

2 Fundamental Differences Between PBS and TBS

2.1 Suitable scope of PBS

PBS is generally more suitable for conventional HRP-DAB IHC workflows. For most paraffin section projects, if antibodies and detection reagents have already achieved stable performance under PBS conditions, PBS can be used as the starting option.

(1) HRP or HRP-polymer detection systems.

(2) Existing methods already showing stable controls under PBS conditions.

(3) Blocking buffer, antibody diluent, and wash solution all using a PBS backbone.

(4) The current major issue is general background control rather than chromogenic system compatibility.

2.2 Suitable scope of TBS

TBS is preferred for AP chromogenic workflows. Once the detection system includes alkaline phosphatase, the wash solution should generally no longer remain PBS. For such workflows, TBS is a foundational condition.

(1) AP chromogenic detection.

(2) Antibody datasheets or validated conditions explicitly specifying a Tris system.

(3) Blocking buffer, antibody diluent, and amplification system already using a Tris backbone.

(4) Existing similar projects already having a TBS-based workflow, where batch consistency needs to be maintained.

2.3 Criteria for switching between PBS and TBS

(1) Whether the detection system requires a Tris backbone.

(2) Whether the datasheet or existing method already defines the system clearly.

(3) Whether the current problem comes from system compatibility, rather than antibody concentration, blocking conditions, or chromogen development time.

Table 1. Decision logic for choosing PBS or TBS

3 The Logic of Enhanced Washing with PBST and TBST

3.1 Changes after adding Tween-20

PBST and TBST are washing systems containing surfactant, formed by adding Tween-20 to PBS or TBS. After Tween-20 is added, liquid spreading over the section surface, tissue wetting, and removal efficiency of hydrophobic residues are all improved.

Common changes include:

(1) More uniform surface coverage of the section.

(2) Easier removal of secondary antibodies, polymer amplification reagents, and proteinaceous residues.

(3) Easier reduction of local deposition at edges, clefts, and luminal spaces.

3.2 Situations suitable for adding Tween-20

Systems containing Tween-20 are more suitable for addressing problems of excessive residual reagent, commonly seen in the following cases:

(1) Negative regions remain insufficiently clean after the secondary antibody step.

(2) Faint deposition appears in blank areas after polymer amplification.

(3) Uneven chromogenic development occurs at section edges or tissue clefts.

(4) Tissues rich in necrotic components, blood components, or mucinous matrix.

(5) Residual-type background is also visible in negative controls.

3.3 Situations where Tween-20 should not be prioritized

The following situations are better first evaluated with PBS or TBS, rather than starting immediately with PBST or TBST:

(1) The antigen is weakly expressed and the positive signal is close to the detection threshold.

(2) The primary antibody has only moderate affinity and the complex is not sufficiently stable.

(3) Membrane proteins, cell surface markers, or fine localization markers.

(4) Frozen sections, fragile tissues, or sections with inadequate adhesion.

(5) The pretreatment has already been relatively strong, and further strengthening washing is more likely to reduce signal.

4 Experimental Positioning of the Four Washing Systems

4.1 Functional division

PBS is mainly used for routine HRP-IHC basic washing.

TBS is mainly used for AP workflows and Tris-based workflows.

PBST is used within PBS-based systems to address excessive residue and insufficient wetting.

TBST is used within TBS-based systems to further enhance residual removal.

4.2 Choosing a system according to the experimental problem

When different problems are encountered, the priority of the four systems is not the same.

(1) For HRP-DAB workflows, start with PBS.

(2) For AP chromogenic workflows, start with TBS.

(3) The basal system has been established, but obvious residual background remains; consider using PBST or TBST.

(4) If the main issue is weak positivity or unstable fine localization, retain PBS or TBS first and do not rush to add Tween-20.

Table 2. Positioning comparison of PBS, TBS, PBST, and TBST

5 Selection Strategies for IHC Workflows

5.1 Determine the basic buffer backbone first

(1) First check the detection system.

(2) Then check the datasheet or existing workflow.

(3) Only then assess the current background pattern.

If the workflow is AP-based, prioritize TBS. If it is a conventional HRP-DAB workflow, prioritize PBS. Do not simultaneously switch PBS/TBS and introduce Tween-20 before the basic backbone is clearly established.

5.2 Then determine whether Tween-20 is needed

(1) Diffuse light staining in negative regions: prioritize a Tween-20-containing system.

(2) Edge accumulation and residue in clefts: prioritize a Tween-20-containing system.

(3) Deposition in necrotic regions and blood-rich regions: Tween-20-containing systems are usually more effective.

(4) Reduced weak positive signal and lighter boundaries: first revert to a system without Tween-20.

5.3 Local reinforcement is preferable to full-process reinforcement

(1) If the background is high after the primary antibody step, first strengthen washing after the primary antibody.

(2) If the background is high after the secondary antibody step, focus on strengthening washing after the secondary antibody.

(3) If deposition is obvious after amplification, strengthen washing between amplification and chromogenic development.

6 Recommended Decision Paths in Experiments

6.1 Conventional HRP-DAB paraffin-section IHC

(1) First establish the basic workflow using PBS.

(2) Observe the background after the secondary antibody step and before chromogenic development.

(3) If the background is mainly residual-type light staining, change the key steps to PBST.

(4) If positive signal becomes obviously lighter after PBST, return to PBS and check blocking, primary antibody concentration, and chromogen development time.

6.2 AP chromogenic IHC

(1) First complete the full workflow using TBS.

(2) Confirm that positive signal is present and reasonably localized.

(3) If obvious residual background remains in negative regions, then evaluate TBST locally.

(4) If signal becomes lighter after TBST, prioritize shortening the enhanced washing step rather than continuing to increase washing strength.

6.3 Low-abundance antigens and weak positive markers

(1) Start with PBS or TBS without Tween-20.

(2) Keep washing time moderate and avoid vigorous agitation.

(3) Focus on comparing the number of positive cells, localization boundaries, and background in negative regions.

(4) If the signal is weak, first review antigen retrieval, primary antibody concentration, and incubation time rather than strengthening washing first.

6.4 Complex samples with high background

For tumor tissues, inflamed tissues, samples with obvious necrosis, blood-rich samples, or mucinous tissues, the following path may be used:

(1) First determine whether the basic backbone should be PBS or TBS.

(2) Then introduce PBST or TBST at key steps.

(3) Focus on strengthening washing after the secondary antibody step and after amplification.

(4) If the negative control remains high, review blocking, antibody specificity, and pretreatment, rather than repeatedly adjusting only the washing solution.

Table 3. Selection of washing systems in common scenarios

7 Common Misunderstandings in Washing-System Optimization

7.1 Attributing all background to insufficient washing

If the background is caused by insufficient blocking, inadequate endogenous enzyme treatment, overly high antibody concentration, or excessively long chromogen development, simply replacing PBS, TBS, PBST, or TBST usually will not fundamentally solve the problem. The background source should be identified first, and only then should the washing system be adjusted if appropriate.

7.2 Using Tween-20-containing systems as the default configuration

PBST and TBST are effective for removing residue, but they are not always favorable for weak-signal projects. For low-abundance antigens, borderline-positive markers, and fine localization studies, the risk of signal loss should always be evaluated before deciding whether Tween-20 should be introduced.

7.3 Changing multiple variables at once

If the basic buffer backbone, the presence or absence of Tween-20, washing time, and agitation strength are all changed simultaneously, the result usually becomes difficult to interpret. A more robust sequence is:

(1) First determine whether PBS or TBS is appropriate.

(2) Then determine whether Tween-20 should be added.

(3) Finally, fine-tune the washing time and the number of wash cycles.

7.4 Comparing only staining intensity

The quality of a washing system should not be judged only by staining depth, but also by:

(1) Whether positive localization matches histological expectations.

(2) Whether negative areas are clean.

(3) Whether cell boundaries are sharp.

(4) Whether abnormal deposition appears in necrotic areas, around vessels, or in stromal regions.

(5) Whether reproducibility is maintained among sections within the same batch.

8 Products Related to IHC Washing Systems

8.1 Product table for basic buffer-system components

8.2 Product table for ready-to-use and premixed wash solutions

There is no fixed priority among PBS, TBS, PBST, and TBST independent of the experimental context. In HRP-DAB workflows, PBS should be evaluated first. In AP workflows, TBS should be evaluated first. When residual-type background is obvious, PBST or TBST can then be considered. For weak positive signals and fine localization projects, washing strength should first be controlled before deciding whether Tween-20 should be introduced. Adjusting in this order provides a clearer optimization path and more reproducible results.

For more related articles, please see below:

[1] Analysis and handling of IHC common problems

[2] Immunohistochemistry (fluorescence) assay

[3] Steps of paraffin section immunohistochemistry