Fluorescence-activated cell sorting (FACS) is a specialized type of flow cytometry. It provides a method for sorting a heterogeneous mixture of cells, one at a time, based upon the specific light scattering and fluorescent characteristics of each cell. It provides fast, objective, and quantitative recording of fluorescent signals from individual cells, as well as physical separation of cells of particular interest. Using the light scattering, FACS can detect cell size and complexity, while the fluorescence characteristics allow the researchers to analyze different biological functions or study specific cell components.

As a flow cytometry technique, FACS is also based on three main systems: fluidics, optics, and electronics. Their combined work allows the process of cell sorting. You can have a look at a schematic flow chart of the whole technique in Figure 1.

Figure 1. FACS schematic flow chart. Sheath fluid is mixed with the sample containing cells in the flow chamber, creating a uniform stream that intercepts a laser beam trajectory generated by the excitation optics in the so-called interrogation point. The generated light scattering and emitted fluorescence is detected by the collection optics and sent to the software controlling the equipment by the electronics system, which controls how to use the deflection plates in order to discard those unnecessary cells in the waste or collect the cells of interest.