Building on Wallace H. Coulter's impedance counter (1953), Mack Fulwyler's droplet cell sorter (1965), Wolfgang Göhde's fluorescence-based pulse cytophotometer (1968), and Leonard Herzenberg's fluorescence-activated cell sorter (FACS), flow cytometry analyzes thousands of single cells per second.
Hydrodynamic sheath fluid focuses suspended cells to pass single-file through one or more laser beams; forward-scattered light (FSC) reports cell size, side-scattered light (SSC) reports cytoplasmic granularity, and dichroic mirrors route fluorophore-conjugated antibody emissions to detector arrays.
In immunopharmacology and oncology, polychromatic and spectral flow cytometers profile T-cell checkpoint receptor occupancy (PD-1, CTLA-4), apoptosis (Annexin V / propidium iodide), cell-cycle DNA ploidy, and intracellular cytokine signaling.