The second major advance was the introduction of the flow cytometer, which measures the fluoroscence of each labeled antibody. The different cell populations are aspirated into the machine, which forces the cells to flow through the chamber singly past a laser beam and light sensors. Light emitted by the excited fluorescent dye on the cell surface is detected by the sensors and analyzed by computer software. Using this system, one can divide cells into different population, depending on their size and granularity. Once the different cell populations are identified, a specific population can be “gated” for further study, such as identification of helper cell (CD4) and supressor cells (CD8-PE), as well as many other lymphocyte subsets.