Flow cytometry

Flow cytometry is a technique for analyzing cells one at a time as they flow past a laser. A suspension of cells, for example white blood cells from a blood sample, is first stained with antibodies tagged with fluorescent dyes, each antibody recognizing a particular protein on or inside the cell. The sample is then forced through a narrow channel in a fluid stream so that the cells line up in single file and cross one or more laser beams, often thousands every second.

Detectors record how each cell changes the light. Light scattered forward, roughly in line with the beam, reflects cell size, while light scattered to the side reflects internal complexity such as granules. Other detectors, behind color filters, measure the fluorescence from each dye, showing which labeled proteins a cell carries and how much. Software plots the results as histograms or dot plots and lets the user gate, or select, populations such as CD4 helper T cells. A related instrument, the fluorescence-activated cell sorter, uses the same measurements to steer chosen cells into separate tubes for further study.

Flow cytometry is used to count CD4 T cells in people with HIV, to classify leukemias and lymphomas by their surface markers, and in research on the cell cycle, apoptosis and stem cells.