Counting cells under a microsope is a bit like trying to count a flock of frightened sheep running around in a field. Like sheep, the best way of counting cells is to align them in single file and get them to pass through a point where they can be counted one at a time. This is the fundamental principle of a flow cytometer, an essential instrument in any immunology lab.
Like many great scientific tools, the history of flow cytometry involves several inventive steps each emerging in different places at different times. For instance, the first patent on a “means for counting particles suspended in a fluid” was filed by American electrical engineer Wallace Coulter in 1949. He invented a way of counting red blood cells in suspension as they passed through a constricted aperture. As a cell travelled through the aperture, it changed the electrical characteristics across the gap, and so could be detected and counted automatically by machine.
Flow cytometry went on in the 1950s to exploit the principle of laminar flow, when fluid flows in parallel layers with no interruption between them, and droplet formation, when a stream of fluid emerging from an aperture is hydro-dynamically unstable and breaks into a series of droplets – the same principle of physics used in inkjet printers.
In the 1960s another important development was the use of fluorescence-based flow cytometers, originally developed by Wolfgang Gohde [umlaut over o] of the University of Munster in 1968. It was known as the ICP 11 device and used light fluorescence to detect the presence of certain cells or particles in suspension. Each particle passing through the light – now invariably laser beams – scatter the light in some pre-determined direction depending on what’s being counted. Fluorescent chemicals found in the particle or attached to the cell can be excited into emitting light at a longer wavelength than the light source, and so be counted and even sorted into different streams. Len Herzenberg, an immunologist at Stanford University, was a pioneer of this method of sorting cells using the principles of flow cytometry. He coined the terms FACS – florescence activated cell sorter – which sorted cells as well as counting them.
The original name for flow cytometry was pulse cytophotometry. So, if there was one key moment in the field it probably came in 1978 when, at the Conference of the American Engineering Foundation in Pensacola, Florida, it was agreed to change the name to “flow cytometry” – counting cells while they move, just like sheep.