Flow Cytometry: Principles, Applications, and Data Analysis

Flow cytometry simultaneously measures multiple physical and fluorescent parameters on single cells in suspension at high throughput (10,000–50,000 events/sec).

Core Principles

• Hydrodynamic Focusing: Sample core centered in sheath fluid → single-file interrogation.
• Light Scatter: FSC (forward scatter ≈ size), SSC (side scatter ≈ granularity/complexity).
• Fluorescence: Fluorochrome-conjugated antibodies bind target antigens; lasers excite; detectors capture emission (bandpass filters).
• Compensation: Spectral overlap correction between fluorochromes.
• Gating: Sequential plots isolate populations of interest (e.g., lymphocytes → singlets → CD3+ → CD4/CD8).

Key Applications

Field	Uses
Immunophenotyping	Lymphocyte subsets (T/B/NK), activation markers, Tregs, exhaustion (PD-1, TIM-3)
Hematologic Malignancies	Leukemia/lymphoma diagnosis (CD markers), MRD (10⁻⁴–10⁻⁶), lineage assignment
Cell Cycle / Proliferation	Ki-67, DAPI/PI, BrdU/EdU — S/G2/M fraction
Apoptosis	Annexin V / 7-AAD or PI — early/late apoptosis
Functional	Cytokine capture (ICS), phospho-flow (pSTAT, pERK), calcium flux, ROS, mitochondrial potential
Cell Sorting (FACS)	High-purity isolation for genomics, culture, transplant

Modern Advances

• Spectral cytometry (40+ parameters, autofluorescence unmixing)
• Mass cytometry (CyTOF) — metal-tagged antibodies, 40+ params, no spectral overlap
• Imaging flow cytometry (morphology + fluorescence)
• Single-cell multiomics (CITE-seq, REAP-seq: surface protein + transcriptome)

Quality Control

• Daily QC beads (alignment, sensitivity, fluorescence stability)
• Standardization: EuroFlow, ICCS, CYTOF panels.
• Biosafety: BSL-2 for unfixed human samples; fixation for shipping.