Fixation and permeabilization buffers for flow cytometry

Fixation and permeabilization buffers are essential for intracellular flow cytometry staining, enabling antibodies to access targets located inside the cell. While surface staining can be performed on live or fixed cells, intracellular staining requires cells to be fixed and permeabilized to allow antibody penetration.

Surface vs intracellular staining

Surface staining

Performed on live or lightly fixed cells, allowing detection of proteins expressed on the cell membrane without disrupting cell integrity.

Intracellular staining

Requires fixation and permeabilization to allow antibodies to access intracellular targets such as cytokines, transcription factors, and phosphorylated proteins.

Fixation buffers

Fixation buffers preserve cell morphology and stabilize proteins by crosslinking cellular components.

• Commonly formaldehyde-based
• Used prior to permeabilization
• Help maintain antigen structure for reliable detection

Permeabilization buffers

Permeabilization buffers disrupt the cell membrane to allow antibody access to intracellular targets.

• Detergent-based (eg saponin or Triton X-100)
• Alcohol-based (eg methanol for phospho-protein detection)
• Choice depends on target location and assay requirements

Protocol overview (quick guidance)

1. Perform surface staining (if required) on live cells
2. Add fixation buffer and incubate to stabilize cellular proteins
3. Wash cells to remove excess fixative
4. Add permeabilization buffer to allow antibody entry
5. Incubate with intracellular antibodies
6. Wash and proceed to acquisition

Blocking buffers for flow cytometry

Blocking buffers are used in flow cytometry to reduce non-specific antibody binding and improve staining specificity. By minimizing background signal, they help ensure accurate and reproducible detection of target antigens.

Common blocking strategies

BSA-based blocking buffers

Bovine serum albumin (BSA) is commonly used to block non-specific binding sites. It is a simple, consistent option suitable for many standard flow cytometry assays.

Serum-based blocking buffers

Serum (e.g. fetal bovine serum or species-matched serum) can provide more complex blocking by saturating potential binding sites. This approach is often used when background staining is higher or more variable.

Fc receptor blocking

In immune cell populations, non-specific binding can occur via Fc receptors, leading to false-positive signals.

Use Fc receptor blocking reagents when working with:

• Peripheral blood mononuclear cells (PBMCs)
• Macrophages and monocytes
• Other immune cell populations

Fc blocking helps prevent antibodies from binding non-specifically via Fc interactions rather than antigen recognition.

When to use blocking buffers

• Prior to antibody staining to reduce background signal
• When working with complex or heterogeneous samples
• In experiments requiring high sensitivity or low background

Basic surface staining workflow

Surface staining is a core flow cytometry workflow used to detect cell surface markers. A simple protocol can help ensure consistent and reliable results.

1. Prepare cells
   Resuspend cells in staining buffer and keep on ice to preserve viability.
2. Block non-specific binding
   Add blocking buffer or Fc receptor blocker to reduce background staining.
3. Add fluorophore-conjugated antibodies
   Incubate cells with surface marker antibodies for 20-30 minutes at 4 degrees Celsius, protected from light.
4. Wash cells
   Wash with staining buffer to remove unbound antibodies.
5. Add viability dye (optional)
   Include a viability dye to exclude dead cells if required.
6. Acquire data
   Resuspend cells in buffer and analyze using a flow cytometer.

Basic intracellular staining workflow

Intracellular staining workflows build on fixation and permeabilization steps to enable detection of targets such as cytokines, transcription factors, and phosphorylated proteins.

1. Prepare cells
   Resuspend cells in staining buffer and keep on ice.
2. Perform surface staining (optional)
   Stain surface markers before fixation if required.
3. Fix cells
   Add fixation buffer and incubate to preserve cell structure and stabilize proteins.
4. Permeabilize cells
   Treat cells with permeabilization buffer to allow antibody access to intracellular targets.
5. Add intracellular antibodies
   Incubate with fluorophore-conjugated antibodies specific to intracellular markers.
6. Wash and resuspend
   Wash cells to remove excess antibody and resuspend in buffer.
7. Acquire data
   Analyze samples on a flow cytometer

Compensation beads for flow cytometry

Compensation beads are used in flow cytometry to generate single-stained controls for accurate fluorescence compensation. They provide a consistent and reliable alternative to cells, particularly when target antigen expression is low or variable.

Why compensation beads are used

• Enable accurate compensation of spectral overlap between fluorophores
• Provide consistent staining signals compared to biological samples
• Reduce variability when preparing single-stain controls

Single-stain control guidance

Compensation beads are commonly used to prepare single-stained controls for each fluorophore in a panel.

• Stain beads individually with each antibody-fluorophore conjugate
• Use one bead sample per fluorophore
• Include an unstained control where required
• Run controls alongside experimental samples to calculate compensation settings

Compatible fluorophores

Compensation beads are available for use with a wide range of commonly used fluorophores, including:

• FITC
• PE
• APC
• PerCP and PerCP-Cy5.5
• Alexa Fluor dyes
• Brilliant Violet dyes

Flow cytometry data is typically visualized using dot plots and histograms, allowing clear identification of cell populations based on fluorescence intensity. For example, compensation controls and single-stained samples help define population boundaries and correct for spectral overlap, ensuring accurate gating and interpretation of multicolor experiments.

Cell viability dyes for flow cytometry

Cell viability dyes are used in flow cytometry to distinguish live and dead cells, ensuring accurate analysis by excluding compromised or non-viable cells. These dyes are essential for improving data quality, particularly in complex or sensitive assays.

Common types of viability dyes

DNA-binding dyes (membrane-impermeable)

These dyes only enter cells with compromised membranes, making them ideal for identifying dead cells.

• Propidium iodide (PI) – widely used for general viability assessment
• 7-AAD – commonly used in multicolor panels and can be useful where its emission profile fits the panel design
• DAPI – suitable for UV-excited systems and rapid viability assessment

Fixable viability dyes (amine-reactive dyes)

These dyes covalently bind to cellular proteins and can be used in workflows that include fixation and permeabilization.

• Enable live/dead discrimination after fixation
• Ideal for intracellular staining protocols
• Available in multiple fluorescence channels for panel flexibility

When to use different viability dyes

Live cell analysis (no fixation)

Use membrane-impermeable dyes such as propidium iodide or 7-AAD

Fixed or intracellular staining workflows

Use fixable viability dyes to retain viability information after fixation

Multicolor flow cytometry panels

Select dyes with minimal spectral overlap

Apoptosis assays for flow cytometry

Apoptosis assays are commonly used in flow cytometry to assess cell health and identify early and late stages of programmed cell death. These assays enable researchers to distinguish viable, apoptotic, and necrotic cell populations, supporting studies in immunology, cancer research, and drug development.

Common apoptosis detection methods

Annexin V assays

Annexin V binds to phosphatidylserine exposed on the outer leaflet of the cell membrane during early apoptosis.

• Detect early apoptotic cells
• Often combined with viability dyes
• Enables clear discrimination between live, early apoptotic, and late apoptotic/necrotic cells

Caspase activity assays

Caspase assays measure the activation of proteases involved in the apoptotic pathway.

• Detect active caspase enzymes
• Useful for confirming apoptosis at a mechanistic level
• Often used alongside Annexin V for comprehensive analysis

Mitochondrial membrane potential dyes

These dyes assess changes in mitochondrial function, which occur early in apoptosis.

• Detect loss of mitochondrial membrane potential
• Useful for identifying early apoptotic events
• Can be combined with other apoptosis markers for multi-parameter analysis

When to use apoptosis assays

• Studying cell death pathways and mechanisms
• Evaluating drug-induced cytotoxicity
• Monitoring immune cell responses
• Investigating disease progression and treatment response

Cell proliferation assays for flow cytometry

Cell proliferation assays are used in flow cytometry to measure cell division and track population expansion over time. These assays are widely applied in immunology, cancer research, and drug development to assess cell growth, activation, and response to treatment.

Common proliferation assays

CFSE

CFSE is a fluorescent dye that passively diffuses into cells and binds intracellular proteins.

• Signal intensity halves with each cell division
• Enables tracking of multiple generations
• Commonly used for lymphocyte proliferation studies

BrdU incorporation assays

BrdU is a thymidine analogue incorporated into newly synthesized DNA during cell division.

• Detects DNA synthesis during the S phase
• Requires DNA denaturation for antibody detection
• Suitable for analyzing actively proliferating cells

EdU assays

EdU is an alternative to BrdU that uses click chemistry for detection.

• Detects DNA synthesis without DNA denaturation
• Faster and simpler workflow compared to BrdU
• Compatible with multicolor flow cytometry panels

When to use proliferation assays

• Measuring immune cell activation and expansion
• Evaluating drug effects on cell growth
• Studying cell cycle progression
• Monitoring tumor cell proliferation

Cell cycle assays for flow cytometry

Cell cycle analysis by flow cytometry enables the measurement of DNA content to determine the distribution of cells across different phases of the cell cycle (G0/G1, S, and G2/M). This approach is widely used to study cell proliferation, cell cycle progression, and the effects of drugs on cell division.

Common cell cycle analysis methods

Propidium iodide (PI) / RNase staining

PI is a DNA-binding dye commonly used for cell cycle analysis.

• Measures total DNA content in each cell
• Requires RNase treatment to remove RNA and ensure accurate DNA quantification
• Enables identification of G0/G1, S, and G2/M populations

BrdU / EdU incorporation assays

These methods measure DNA synthesis during the S phase.

• Detect actively replicating cells
• Often combined with DNA dyes for dual-parameter analysis
• EdU assays offer faster detection without DNA denaturation compared to BrdU

When to use cell cycle assays

• Analyzing cell cycle progression and checkpoints
• Evaluating drug-induced cell cycle arrest
• Studying cancer cell growth and regulation
• Monitoring cell proliferation in combination with DNA content analysis

Spectral flow cytometry reagents

Spectral flow cytometry is an advanced approach that captures the full emission spectrum of each fluorophore, enabling improved resolution of highly complex, high-parameter panels. Unlike conventional flow cytometry, spectral systems use computational unmixing to distinguish fluorophores with overlapping emission profiles.

As panel complexity increases, selecting compatible reagents and controls becomes critical to ensure accurate spectral unmixing and data interpretation.

Key considerations for spectral flow cytometry

Fluorophore compatibility

Spectral flow allows the use of fluorophores with overlapping emission, but careful selection is still required to ensure accurate unmixing and signal resolution.

Reference controls

High-quality single-stain controls are essential for generating accurate reference spectra used during spectral unmixing.

Reagent consistency

Reproducible staining and consistent reagent performance are critical for building reliable spectral libraries and minimizing variability between experiments.

Reagents used in spectral workflows

• Fluorophore-conjugated antibodies compatible with spectral systems
• Single-stain controls and reference standards
• Viability dyes selected to minimize spectral interference
• Fixation and permeabilization buffers compatible with downstream analysis

Why spectral flow matters

Spectral flow cytometry enables deeper immune profiling, improved resolution of rare populations, and expansion to high-parameter panels, supporting more advanced experimental design and data analysis.

Fluorophore selection and panel design guidance

Effective panel design is critical for successful flow cytometry experiments, particularly in multicolor applications. Selecting the right combination of flow cytometry antibodies and fluorophore conjugates helps minimize spectral overlap, improve signal resolution, and ensure accurate data interpretation.

Key considerations for panel design

Spectral overlap and compensation

Choose fluorophores with minimal emission overlap to reduce the need for complex compensation and improve data clarity.

Antigen expression level

Match bright fluorophores to low-abundance targets, and dimmer fluorophores to highly expressed markers to balance signal intensity across the panel.

Instrument configuration

Ensure all fluorophores are compatible with your cytometer’s laser lines and detector filters.

Panel complexity

As the number of markers increases, careful panel design becomes essential to avoid signal spread, compensation errors, and loss of resolution.

Practical tips

• Use appropriate controls to optimize panel performance
• Include viability dyes to exclude dead cells and improve data quality
• Validate panels with pilot experiments before scaling up

Instrument and laser compatibility

When selecting flow cytometry reagents, it is important to ensure compatibility with your instrument’s laser configuration and detector setup. Different fluorophores require specific excitation wavelengths and emission filters to generate accurate and reliable signals.

Laser compatibility overview

Common laser lines

Most flow cytometers are equipped with standard laser lines, including:

• 405 nm (violet)
• 488 nm (blue)
• 561 nm (yellow-green)
• 633-640 nm (red)

Fluorophore matching

Fluorophores must be matched to the appropriate laser for excitation:

• FITC, PE – excited by 488 nm laser
• APC – excited by 633 – 640 nm laser
• Brilliant Violet dyes – excited by 405 nm laser

Reagent compatibility considerations

Fluorophore-conjugated antibodies

Ensure compatibility with your instrument’s lasers and detector filters to achieve optimal signal detection.

Viability dyes

Select dyes that align with available laser lines and minimize spectral overlap within your panel.

Compensation beads and controls

Should be matched to the fluorophores used in your panel to ensure accurate compensation and unmixing.

Practical tip

Always check your cytometer configuration when designing panels to ensure all reagents can be properly excited and detected.

Flow cytometry reagent selection guide by application

Selecting the right reagents depends on your experimental application and workflow. The guide below outlines common flow cytometry applications and the key reagent types required to support accurate and reliable results

Clinical vs research use in flow cytometry reagents

Flow cytometry reagents are used across a range of applications, from basic research to clinical and translational studies. Understanding the intended use of reagents is important when selecting the right products for your workflow.

Research use reagents

• Designed for basic and exploratory research applications

• Used in areas such as:

  • Immunology
  • Cell biology
  • Cancer research

• Typically labeled for research use only (RUO)

• Suitable for assay development, method optimization, and discovery workflows

Clinical and translational research applications

• Used in clinical research settings to study disease mechanisms and biomarkers

• Common in:

  • Immunophenotyping of patient samples
  • Monitoring treatment response
  • Translational and preclinical studies

• Require high consistency and reproducibility across experiments

Important considerations

Flow cytometry reagents are not intended for diagnostic use. Always check product specifications and regulatory status before use in clinical or diagnostic workflows.

Access our complete flow cytometry guide