Fluorescence-activated cell sorting (FACS) is a specialized type of flow cytometry. It provides a method for sorting a heterogeneous mixture of biological cells into two or more containers, one cell at a time, based upon the specific light scattering and fluorescent characteristics of each cell. It is a useful scientific instrument, as it provides fast, objective and quantitative recording of fluorescent signals from individual cells as well as physical separation of cells of particular interest.
This method enables researchers to better understand the characteristics of a single cell population without the influence of other cells. Compared to other methods of cell enrichment, such as magnetic-activated cell sorting (MCS), FACS is more flexible and accurate for cell separation due to the ability of phenotype detection by flow cytometry. In addition, FACS is usually capable of separating multiple cell populations simultaneously, which improves the efficiency and diversity of experiments. Although FACS has some limitations, it has been broadly used to purify cells for functional studies in both in vitro and in vivo settings. Different cell types usually express unique molecules, or a unique combination of several molecules, on the plasma membrane that can distinguish one cell population from another. Upon binding of these cell surface molecules by specific fluorescence-conjugated antibodies, a detecting machine called flow cytometer/sorter is able to excite and detect the light signals of different fluorescent dyes that represent different molecule markers on the cells at the single cell level. The combined information consisting of either the presence of a light signal (representing positive expression of the corresponding surface molecule) or the absence of a light signal (representing negative expression of a molecule) defines the phenotype of the cell. After passing through the detector, cells with the same phenotype of interest are diverted towards a designated collecting tube based on electrical charge.

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