Cell in laser beam, flow cytometry. Illustration of a white blood cell (leukocyte) passing through the laser beam in a flow cytometer. Flow cytometry is a technology which revolutionized the fields of immunology and cell biology as it allows large numbers of cells to be rapidly analyzed. Cells travel through the flow cytometer in a stream of fluid which aligns them so that they pass through the laser beam in single file. Specific cells can then be counted, analyzed, and sorted out from within a mixed population of different cell types. This technology is widely used in science research, diagnosing diseases, monitoring treatments and clinical trials. Supplied by Neil Dufton, courtesy of Wellcome Images.