Key takeaways

• Forward scatter correlates with particle size and side scatter with internal granularity; together they separate cell types and distinguish debris from intact cells.
• Detector gain is set on PMTs — photomultiplier tubes. Setting these correctly with unstained and fully stained controls comes before anything else.
• Match fluorochrome brightness inversely to antigen density: bright fluorochromes for low-abundance antigens, dim ones for highly expressed targets.
• Single-stained controls drive compensation; FMO controls set gates, because spillover from other channels shifts where the negative boundary truly sits.
• An isotype control must match the primary antibody’s species, isotype and fluorochrome — a human IgG1 control tells you nothing about a mouse IgG1 antibody.
• Always include a viability dye. Dead cells bind antibody non-specifically and generate false positive populations.
• Titrate every antibody. Using the manufacturer’s suggested volume without titration is the commonest cause of poor signal-to-noise.

Contents

• What flow cytometry measures
• Forward and side scatter
• Protocol 1: cell cycle analysis by PI staining
• Protocol 2: surface receptor expression
• Controls reference table
• Compensation and spillover
• A standard gating order
• Ten practical tips
• Choosing reagents
• Frequently asked questions

Reagents for flow cytometry

A workable panel needs conjugates across separate detectors, a properly matched isotype control, and a viability dye — all three represented below.

CD3-FITC

FITC Anti-Human CD3 Antibody [OKT-3]

FITCHuman

Pan-T lineage marker on the FITC detector; the anchor gate for most lymphocyte panels.

View product →

CD4-PE

PE Anti-Human CD4 Antibody [RPA-T4]

PEHuman

Helper T cell marker; PE is bright, so it suits lower-density antigens.

View product →

CXCR4

APC Anti-Human CD184 / CXCR4 Antibody [12G5]

APCHuman

Surface receptor measured in Protocol 2; APC sits on the red laser, minimising spillover with FITC.

View product →

Isotype ctrl

PE Mouse IgG1, kappa Isotype Control [MOPC-21]

PEMouse IgG1

Matches the species, isotype and fluorochrome of the PE-conjugated mouse antibodies above.

View product →

Propidium iodide

Propidium Iodide Staining Solution

DNA stainFlow cytometry

Used for DNA content in Protocol 1, and as a viability dye in surface staining panels.

View product →

7-AAD

7-AAD Viability Staining Solution

Viability dyeFlow cytometry

Excluded by intact membranes; separates live from dead cells without the spectral breadth of PI.

View product →

What flow cytometry measures

The name describes the method: flow for the moving stream, cyto for cells, metry for measurement. Cells in suspension pass single file through one or more laser beams, and detectors record scattered and emitted light from each event individually.

That single-cell resolution at high throughput — hundreds of events per second — is what distinguishes it from bulk assays. A western blot averages across a lysate; flow cytometry tells you what proportion of cells carry a marker and how much each one carries. It supports cell counting, sorting, biomarker detection, cell cycle analysis and apoptosis measurement.

Forward and side scatter

Two parameters are collected on every event before fluorescence is considered at all:

• Forward scatter (FSC) — light scattered at a shallow angle, correlating with particle size. Used to separate intact cells from smaller debris.
• Side scatter (SSC) — light scattered at ninety degrees, reflecting internal complexity and granularity.

Plotted together, FSC and SSC separate the major leukocyte populations without any staining at all — lymphocytes low on both, monocytes intermediate, granulocytes high on side scatter. Every gating strategy starts here.

Protocol 1: cell cycle analysis by propidium iodide staining

Fixes cells and stains DNA so that G1, S and G2/M populations can be resolved by DNA content.

Reagents and equipment

• Pipettes and tips
• Ethanol, 70% (v/v), ice-cold
• RNase A
• Phosphate buffered saline (PBS)
• Propidium iodide
• A flow cytometer

Fixation

• Harvest cells by centrifugation at 270 × g for 5 min to pellet.
• Carefully aspirate the supernatant without disturbing the pellet.
• Wash the pellet in ice-cold PBS.
• Centrifuge at 270 × g for 5 min.
• Resuspend in 200 µl PBS.
• Add 2 ml ice-cold 70% (v/v) ethanol dropwise while vortexing gently, then incubate at 4 °C overnight before analysis.

DNA staining

• Centrifuge fixed samples at 270 × g for 5 min.
• Aspirate the ethanol and resuspend in 500 µl PBS.
• Add RNase A and propidium iodide, and mix.
• Incubate in the dark at 37 °C for 30 min before analysis.

Why RNase is not optional: propidium iodide intercalates double-stranded RNA as well as DNA. Without RNase digestion the DNA content histogram is broadened and the cell cycle phases cannot be resolved cleanly.

Adding ethanol dropwise with gentle agitation also matters — adding it in bulk causes clumping, and clumps become the doublets discussed in tip 6.

Protocol 2: surface receptor expression

Measures the effect of a treatment on cell surface receptor levels — here CXCR4 on Jurkat T cells following SDF-1 alpha stimulation.

Reagents and equipment

• Pipettes and tips
• RPMI with 0.5% BSA
• SDF-1 alpha at 100 ng/ml
• Phycoerythrin (PE)-labelled antibodies against the receptor of interest
• PBS with 1% BSA
• 1% paraformaldehyde
• A flow cytometer

Stimulation and staining

• Seed Jurkat T cells at 5 × 10⁵ cells/ml and serum starve for 2 hours in serum-free RPMI with 0.5% BSA.
• Stimulate with 100 ng/ml SDF-1 alpha under discontinuous stirring.
• Stop the reaction by centrifugation for 30 sec at 850 × g on a benchtop centrifuge.
• Wash cells with ice-cold PBS.
• Stain with PE-labelled antibodies in 1% BSA/PBS and incubate 30 min on ice, protected from light.
• Wash twice with 1% BSA/PBS.
• Fix in 1% paraformaldehyde on ice.
• Store up to 24 hours at 4 °C protected from light, or analyse immediately.

Staining on ice throughout is deliberate: surface receptors internalise at 37 °C, so warm staining underestimates surface expression — which is precisely the measurement being made here.

Controls reference table

Controls fall into three groups: instrument set-up controls for voltages and compensation, gating controls to separate specific from non-specific signal, and experimental controls for biological meaning.

On isotype controls specifically: they are less informative than often assumed, and an FMO control is the better tool for setting a gate, because it accounts for spillover from every other channel in the panel rather than for non-specific binding alone. Where an isotype is used, it must match the primary antibody in species, isotype, fluorochrome and ideally protein-to-fluorochrome ratio.

Compensation and spillover

Fluorochromes emit across a range of wavelengths rather than at a single one, so light from one fluorochrome reaches detectors intended for others. That overlap is spillover, and correcting for it arithmetically is compensation.

• Compensation is calculated from single-stained controls — one per fluorochrome, on the same cells or on capture beads.
• The single-stained control must use the same fluorochrome as the panel, since spillover is a property of the dye, not the antibody.
• Compensation corrects the mean but cannot remove the added variance — spillover widens the spread of the data, which is why FMO controls remain necessary for gating even after compensation.
• Brighter is not always better: a very bright fluorochrome in a channel adjacent to a dim one can make the dim marker uninterpretable.

A standard gating order

Order matters. Gating a marker before excluding dead cells and doublets inflates the apparent positive fraction, and no amount of downstream analysis recovers from it.

Ten practical tips

1. Optimise sample preparation

This is where most experiments are won or lost. Harvesting must preserve viability, and any enzymatic dissociation must not cleave the epitopes being measured — trypsin removes several surface markers, so a gentler dissociation reagent is often required.

2. Match fixation and permeabilisation to the antigen

No single fixation and permeabilisation combination suits every target. Surface staining is generally done before fixation; intracellular targets need permeabilisation, and nuclear targets need a harsher method than cytoplasmic ones.

3. Choose fluorochromes against your instrument

Know the laser lines, filter sets and emission spectra available before designing a panel — no two cytometers are alike. Assign bright fluorochromes to low-density antigens and dim ones to highly expressed targets, and consider spillover when placing them.

4. Select the right controls

Use instrument controls for voltages and compensation, gating controls to distinguish specific from non-specific signal, and experimental controls for biological interpretation. The table above lists which control serves which purpose.

5. Identify and exclude dead cells

Dead cells bind antibody non-specifically and produce false positives. Include a dye excluded by intact membranes, such as propidium iodide or 7-AAD, and gate them out before analysing markers.

6. Gate out doublets

Doublets are two cells passing the laser together and being recorded as one event, falsely raising fluorescence intensity. Exclude them using height against area, or width against area, and filter samples before acquisition to reduce clumps.

7. Store samples correctly if you cannot run them immediately

Analyse fresh where possible. If not, resuspend in around 100 µl of fixative such as 1–2% formaldehyde and store at 4 °C protected from light for up to 24 hours.

8. Titrate every antibody

Manufacturer-recommended volumes are a starting point, not an answer. Titration — a dilution series scored on separation between positive and negative — improves signal-to-noise, and usually reduces antibody consumption as well.

9. Set compensation with single-stained controls

Use one single-stained control per fluorochrome, prepared with the same dye as the panel and at comparable brightness. Compensation corrects mean spillover but not the variance it adds, so keep FMO controls for gating.

10. Protect from light throughout

Fluorochromes photobleach under ambient light. Cover tubes and plates with foil during staining, incubation and storage, and keep exposure to a minimum during handling.

Frequently asked questions