Biology of Disease

BOX 6.1 The flow cytometer

The flow cytometer (Figure 6.3) is an instrument that can analyze

several properties of cells simultaneously in mixed populations.

The cells to be analyzed are passed as single cells in a stream

past a laser light source. This is usually achieved by having a

sheath of fluid passing through an orifice of 50–300 Mm. The

sample is injected into the sheath fluid as it passes through the

orifice. With the right sheath fluid flow rate, the sample and the

fluid do not mix (Figure 6.4). As the cells are illuminated by the

laser beam some of the light is scattered. The scattered light is

detected simultaneously by two detectors. One measures side

scatter, that is the light deflected 90º from the incident beam.

The other detects the light scattered in a forward direction up

to 10o from the incident beam. This is the forward scatter. The

intensity of the forward and side scatters are related to the size

and shape of the cells. The forward scatter is sensitive to the sur-

face characteristics of the cell, whereas side scatter is more sensi-

tive to the granularity of a cell. Thus a mixed population of cells

can be analyzed on the basis of these measurements. In addition

Figure 6.3A flow cytometer.Obtained from http://flowcyt.cyto.purdue.edu/

flowcyt/educate/photos/flowware/fwarepre.htm

Figure 6.4Schematic of a Flow Cell.See main text for details.

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Focused

laser

beam

Sheath

fluid

Sheath

fluid

Fluorescence

and

scatter signals

Injector tip

that these procedures were successful when the blood transfused was of an

identical blood group. The discovery of the ABO blood group system led to

Landsteiner receiving the Nobel Prize for Physiology and Medicine in 1930.

Landsteiner later discovered other blood group systems, including the Rh sys-

tem. Since then, numerous other systems have been discovered, as shown in