flow cytometry and FACS
/ floh sy-TOM-eh-tree and fakss /
Picture a coin-sorting machine that takes a jumbled jar of coins, sends them past a sensor one at a time, and drops each into the right bin. Flow cytometry does this for cells: it lines up thousands of cells per second into a single-file stream, shines a laser on each one as it passes, and measures what each cell looks like — its size, its internal complexity, and which glowing tags it carries. FACS adds the final step: it physically sorts those cells into separate tubes.
Cells are suspended in fluid and squeezed into a stream so narrow that they pass a laser beam one at a time. As each cell crosses the laser, detectors record how it scatters light (a clue to its size and graininess) and how brightly it glows from any fluorescent tags attached to it — for instance, fluorescent antibodies marking specific surface proteins. This produces a measurement on every single cell, letting you count and characterise a whole population fast. In fluorescence-activated cell sorting (FACS), the machine then charges and deflects droplets containing chosen cells so it can collect, say, only the cells flashing green into a separate tube.
Flow cytometry and FACS are indispensable for studying mixed cell populations: counting how many cells carry a given marker, sorting rare cell types out of a crowd, or isolating pure stem cells or immune-cell subsets for further study. They are fast and quantitative across huge numbers of cells. Their limits: the cells must be free-floating and singular (so solid tissue must first be broken into single cells), and the method reports the presence of labelled markers, not the fine internal structure that a microscope would reveal.
From a blood sample containing many cell types, a FACS machine sorts out only the rare immune cells carrying a specific fluorescent-tagged surface marker, delivering a pure tube of just those cells for study.
Thousands of cells per second pass a laser; FACS can physically separate the chosen ones.
Flow cytometry needs cells floating free and one at a time, so solid tissue must be broken apart first; it measures labelled markers and overall light scatter, not a cell's detailed internal picture.