Flow Cytometry Gating: A Practical Guide to Sequential Gating Strategies

Gating is where flow cytometry data becomes biology. You can have the best panel, the cleanest compensation, and a perfectly maintained instrument — but if your gates are wrong, your results are wrong. The challenge is that gating is part science and part judgment call: there is no universal "correct" gate position, and the right strategy depends on what you are trying to measure, which tissues you are working with, and what controls you have included.

This guide covers the practical logic of gating — how to build a gating hierarchy from scratch, where to place gates for common immune cell populations, and how to use controls to validate your decisions. If you are new to flow cytometry, start here. If you are experienced but want to sharpen your gating, skip to the sequential gating examples below.

Step 1: The Universal Starting Gates

Every flow cytometry gating strategy begins the same way, regardless of what you are staining for. These three gates clean up your data before you touch any fluorescence channels:

1a. Scatter gate (FSC-A vs SSC-A): Plot forward scatter area against side scatter area. Draw a gate around the main cell population to exclude debris (low FSC, low SSC) and dead cells or aggregates at the extremes. This gate removes the events that are not intact, single cells.

1b. Doublet exclusion (FSC-H vs FSC-A or FSC-W vs FSC-A): Two cells passing through the laser together (doublets) will appear as a single bright event, causing false positives in every channel. Plot FSC-Height vs FSC-Area — singlets fall along the diagonal, doublets fall above it. Gate on the singlet population. This step is non-negotiable for any quantitative analysis.

1c. Live/dead exclusion: Add a viability dye (e.g., DAPI, 7-AAD, Live/Dead Fixable dyes) and gate out the dead cells. Dead cells bind antibodies nonspecifically and will contaminate every fluorescence gate downstream. If you skip this step, you are measuring dead-cell artifacts, not biology.

Step 2: Where to Put the Gate — Controls That Tell You

The most common mistake in gating is placing a gate based on where you think the positive population should be. Instead, gates should be placed based on where your controls tell you the negative population ends. Three types of controls are used for this purpose:

In practice, the gold standard for gate placement in multicolor experiments is the FMO control. Place your gate at the upper boundary of the FMO-negative population — everything above that line in your fully stained sample is considered positive. For a detailed protocol on setting up FMO and isotype controls together, see our Isotype Control Flow Cytometry Protocol.

Step 3: Sequential Gating for Common Immune Populations

Once you have clean, live, single cells, the next step is a sequential (hierarchical) gating strategy tailored to the populations you want to identify. Below are three commonly used gating hierarchies:

T Cell Subsets from PBMCs

Myeloid Populations from Tissue

Tumor-Infiltrating Lymphocytes (TILs)

Common Gating Mistakes

Gating on dead cells. Dead cells are sticky — they bind almost any antibody nonspecifically and will appear positive in multiple channels. Always include a viability dye and gate them out before analyzing any markers.

Skipping doublet exclusion. Two cells stuck together will have the fluorescence intensity of both cells combined. A CD4+ T cell stuck to a CD8+ T cell becomes a "CD4+ CD8+" doublet — a population that does not exist biologically but appears regularly in ungated data.

Setting gates without FMO controls. In a 10-color panel, the spectral spillover from 9 other fluorochromes can shift the apparent "negative" boundary of a marker substantially. Without an FMO, you may place a gate that is either too conservative (missing true positives) or too liberal (including spillover-driven false positives).

Drawing gates too tight. Over-gating (excluding borderline events) biases your data toward the brightest cells. Use your FMO to set the boundary, then include everything above it — even the dim positives. The dim population is often the most biologically interesting (e.g., recently activated T cells upregulating an activation marker).

Applying the same gates across different samples. Gate positions can shift between samples due to differences in autofluorescence, staining intensity, or compensation. In high-parameter experiments, consider setting gates on a per-sample basis using each sample's own FMO, especially for tissue-derived samples where autofluorescence varies.

Building Your Flow Cytometry Panel

Good gating starts before you touch the cytometer — it starts with good panel design. A well-designed panel minimizes spectral overlap, assigns the brightest fluorochromes to the dimmest markers, and avoids channel combinations that create gating ambiguity.

abinScience offers flow cytometry antibodies conjugated to commonly used fluorochromes across human, mouse, rat, and other species — including pre-titrated formats optimized for minimal compensation burden.

For research use only. Not intended for diagnostic or therapeutic use.