- Cell size (forward scatter)
- Internal complexity or granularity (side scatter)
- Relative fluorescence intensity.
The light source used in most flow cytometers is laser.
- Leukemias and lympomas: Immunophenotyping (evaluation of cell surface markers), diagnosis, detection of minimal residual disease, and to identify prognostically important subgroups.
- Paroxysmal nocturnal hemoglobinuria: Deficiency of CD 55 and CD 59.
- Hematopoietic stem cell transplantation: Enumeration of CD34+ stem cells.
- Feto-maternal hemorrhage: Detection and quantitation of foetal hemoglobin in maternal blood sample.
- Anemias: Reticulocyte count.
- Human immunodeficiency virus infection: For enumeration of CD4+ lymphocytes.
- Histocompatibility cross matching.
The first fluorescence-based flow cytometry device (ICP 11) was developed in 1968 by Wolfgang Göhde from the University of Münster, Germany and first commercialized in 1968/69 by German developer and manufacturer Partec through Phywe AG in Göttingen. At that time, absorption methods were still widely favored by other scientists over fluorescence methods. The original name of the flow cytometry technology was pulse cytophotometry (German: Impulszytophotometrie). Only 10 years later in 1978, at the Conference of the American Engineering Foundation in Pensacola, Florida, the name was changed to flow cytometry, a term that quickly became popular. Soon after, flow cytometry instruments were developed, including the Cytofluorograph (1971) from Bio/Physics Systems Inc. (later: Ortho Diagnostics), the PAS 8000 (1973) from Partec, the first FACS instrument from Becton Dickinson (1974), the ICP 22 (1975) from Partec/Phywe and the Epics from Coulter (1977/78).
Principle of flow cytometry
A beam of light (usually laser light) of a single wavelength is directed onto a hydrodynamically-focused stream of fluid. A number of detectors are aimed at the point where the stream passes through the light beam: one in line with the light beam (Forward Scatter or FSC) and several perpendicular to it (Side Scatter (SSC) and one or more fluorescent detectors).
Each suspended particle from 0.2 to 150 micrometers passing through the beam scatters the light in some way, and fluorescent chemicals found in the particle or attached to the particle may be excited into emitting light at a longer wavelength than the light source. This combination of scattered and fluorescent light is picked up by the detectors, and, by analyzing fluctuations in brightness at each detector (one for each fluorescent emission peak), it is then possible to derive various types of information about the physical and chemical structure of each individual particle.