A stream of cells flows past a laser. Each cell scatters light and emits fluorescence. The instrument records the signals and sorts the cells based on their properties. That is flow cytometry. It analyzes and sorts individual cells at thousands per second. It is one of the most powerful tools in immunology and cell biology.

The technique works by labeling cells with fluorescent antibodies. Each antibody binds a specific marker on the cell surface or inside the cell. When the cell passes through the laser, the fluorophore is excited and emits light at a characteristic wavelength. The instrument measures the intensity of the light, which tells how much marker is present. It also measures forward scatter, which indicates cell size, and side scatter, which indicates internal complexity.

Flow cytometry is used to diagnose leukemia and lymphoma by identifying abnormal cell populations. It is used to monitor HIV by counting CD4 T cells. It is used to analyze stem cells, sort cell populations for research, and detect rare cells in blood. Cell sorting, called FACS, can physically separate cells into different tubes for further study.

The technology is complex and expensive. It requires trained operators and careful quality control. But it gives information that no other technique can provide: a snapshot of every cell in a population, one by one.