Learning Objective: At the end of this lesson, the learner will be able to explain the principle, process, and clinical applications of flow cytometry and interpret flow cytometry plots used in immunophenotyping and disease diagnosis.
Flow cytometry is a laboratory technique that allows for the quantitative analysis of physical and chemical properties of individual cells in a fluid sample. It is especially valuable in hematology, immunology, and oncology for assessing cell size, granularity, and protein expression (immunophenotype).
Principle
Cells are:
- Labeled with specific antibodies directed against cell-surface or intracellular antigens.
- These antibodies are conjugated with fluorescent dyes, each dye emitting light of a distinct wavelength.
- Cells pass one at a time through a laser beam inside the cytometer.
- The instrument measures:
- Forward scatter (FSC): correlates with cell size
- Side scatter (SSC): correlates with granularity
- Fluorescence intensity: reflects the amount of bound antibody (i.e., protein expression)
Data Output
Results are displayed as:
- Histograms: representing one parameter (e.g., CD4 expression)
- Scatter plots (dot plots): representing two parameters simultaneously (e.g., CD3 vs CD8)
Quadrant Interpretation Example (CD3 vs CD8 Plot)
| Quadrant | CD3 Expression | CD8 Expression | Interpretation |
|---|---|---|---|
| Lower Left | Negative | Negative | Cells lacking both markers |
| Lower Right | Positive | Negative | CD3⁺ T cells without CD8 |
| Upper Left | Negative | Positive | CD8⁺ non-T cells (rare) |
| Upper Right | Positive | Positive | CD8⁺ cytotoxic T lymphocytes |
Clinical Applications
| Field | Application |
|---|---|
| Hematology/Oncology | Diagnosis and classification of leukemia and lymphoma (by immunophenotyping) |
| Immunology | Measurement of CD4⁺ T-cell count in HIV patients |
| Hematologic disorders | Detection of paroxysmal nocturnal hemoglobinuria (PNH) |
| Obstetrics | Detection of fetal red blood cells in maternal circulation |
| Transplant medicine | Monitoring of immune reconstitution or rejection |
Summary Table
| Parameter | Assessed Property | Clinical Use |
|---|---|---|
| Forward Scatter (FSC) | Cell size | Differentiating lymphocytes vs monocytes vs granulocytes |
| Side Scatter (SSC) | Cell granularity | Identifying granulocytes and blasts |
| Fluorescence | Antibody-labeled protein expression | Immunophenotyping (e.g., CD markers) |
